Will there be found ore patches, ore stones (such as coal, stone, zircon or other minerals?
Will spawn Biters?

Highly preciated if so.

Also use some PNG for the cavern. Plenty of resources here https://opengameart.org/
This 1 you can use https://opengameart.org/content/the-dungeon-collection-from-glitch

Thats definitely on my todo list but I dont have ideas yet how to implement it in a balanced manner

Create new types of rocks that spawn different minerals using ratios (for the type of rock, 20% iron, 15% copper for instance, that adds stone and the ore).
You can use the minerals from K2, BZ, Timeken mods. Also for SE only in the planets with specific conditions can spawn rocks with the resources for this planets (See SE Modding interface).

Also orbits in SE should not be diggable.

For the biters, I would pre populate certain areas (with some radious from the last explored space) if there are biters, so there are pockets with biters.

For the ores you could follow a similar approach such as prepopulating.
I hope this helps.

I suggest adding an "ore prospecting" technology that allows you to craft and install a machine in a chunk, which, after a certain amount of work, has a chance to create an ore patch in that chunk, with results based off of the original surface's ore generation settings, and the ore count/yield scaled by current depth (deeper levels have richer patches, but the operation is more expensive). The necessity to check every chunk individually requires more investment and involvement than simply setting a bunch of AAI miners on an endless quest in search of fortunes, and also doesn't create distractions in the form of unmined resources/ore patches on the map if you choose not to use it. You probably cannot append any extra data to the chunk structure as a mod, but you could create a lookup table for already prospected chunks to make it a one-time operation per individual chunk. A square 20k-tile-wide surface only has 390,625 chunks in it, and bit-shifting the flag into a 32-bit integer allows you to compress the table into just 12KB. You could potentially set the initial table size to Min(MaxSurfaceSize, 20000), and for surfaces with max sizes exceeding 20k, you can rescale the table whenever the player tries to prospect a chunk out-of-bounds.

For biters, an approach that makes sense and doesn't necessarily require overwriting the biters' behaviors/bases would be to periodically (based on the map's biter expansion timer settings) check for biter bases one layer above an exposed spot, and should one be found, spawn a biter expansion party directly below it, and set its task to settle on that chunk. They just burrowed underground.

Create new types of rocks that spawn different minerals using ratios (for the type of rock, 20% iron, 15% copper for instance, that adds stone and the ore).
You can use the minerals from K2, BZ, Timeken mods. Also for SE only in the planets with specific conditions can spawn rocks with the resources for this planets (See SE Modding interface).

Good idea. Something like meteorites in SE which drop different resources by percantage?

For the biters, I would pre populate certain areas (with some radious from the last explored space) if there are biters, so there are pockets with biters.
For the ores you could follow a similar approach such as prepopulating.

I don't like the idea of seeing enemy pockets one the map if you didn't dig there (which is like ModMash Unterground). My idea is once you dig where a pocket should be, it will be wholely exposed. This way you can't avoid enemies by digging around (and I don't need to disable radars in subsurfaces).

I suggest adding an "ore prospecting" technology that allows you to craft and install a machine in a chunk, which, after a certain amount of work, has a chance to create an ore patch in that chunk

I wouldn't let the machine spawn resources magically rather scan nearby chunks for ore or enemy occurances.

@Nata: Good idea. Something like meteorites in SE which drop different resources by percantage?
Exactly!
@Natha: I don't like the idea of seeing enemy pockets one the map if you didn't dig there (which is like ModMash Unterground). My idea is once you dig where a pocket should be, it will be wholely exposed. This way you can't avoid enemies by digging around (and I don't need to disable radars in subsurfaces).
I get it, this ixactly what I would not like... would be a nasty surprise... but maybe this is a feature and not a bug! sounds good to me.

I wouldn't let the machine spawn resources magically rather scan nearby chunks for ore or enemy occurances.

The idea is that they are not magically spawned, but rather "revealed" when prospected, and simply not visible from the start. If you combine the map seed with chunk and surface indices for use as PRNG seeds, you can make the ores static per map. If I understand the suggested approach of spawning rocks containing materials correctly, the positions they are spawned at would be random, and they would only presumably be excavatable by AAI miners, which doesn't play nicely with mods that add interactions with ore patches, and it also prevents the use of modules in the process. It would also have a more severe impact on performance due to multiple patrolling vehicles, as well as not allowing for spawning liquid resources, unless you're willing to create solid items and liquefying recipes for all of them.

When considering the addition and implementation of anything to a game, it is wise to first consider every way the player will be able to interact with the new addition, and preemptively dismiss/rework certain ideas if they are likely to negatively impact the enjoyment players get from the game, or make the players intentionally avoid interacting with them. One example of a failed design is the lack of early automated repair/building options in the base game. It is not much of an issue when playing the base game on default settings, however if you, for example, wanted to play with a huge research cost modifier in order to get to enjoy building larger-scale structures from the beginning of the game rather than building a small setup and rushing the QoL technologies, and at the same time, you like considering and setting up defenses against biters and seeing constant attacks, so you also use some aggressive biter modifiers, you're unlikely to have a very enjoyable time, as you're going to be forced into constantly running around the perimeter and repairing broken walls due to lack of other options. When you play with mods, the issue becomes even more apparent, as the overhaul mods not only prolong the game as much as or even more than moderate research cost multipliers, they also require larger spaces for setting up the machinery, which directly affects the effective defensive perimeter length. This causes mods like Repair Turret to become popular, when in reality, this should have been a feature in the base game.

While focusing on making the mod unique might seem appealing as a developer, it ultimately doesn't mean much if the players are put off by the lack of integration with other gameplay elements/ease of use, at least in my personal opinion.

I am now at the point that I can reveal ore patches using the game's generator, based on given frequency, size and richness per resource and per surface. Now I feel stuck. I want to focus on resources, enemies will come later.
I could implement that ore patches are covered by their corresponding "rich stone" as suggested by SrConrabo.

I suggest adding an "ore prospecting" technology that allows you to craft and install a machine in a chunk, which, after a certain amount of work, has a chance to create an ore patch in that chunk

Any concrete ideas what this work could be? Only time and energy seems too simple and boring. And how to automate?

Any concrete ideas what this work could be? Only time and energy seems too simple and boring. And how to automate?

If you want to do something that feels "unique", you could make the machine be a disposable unit that requires sulfuric acid barrels and explosives to operate, and after a set amount of cycles is performed, it creates an explosion centered on the chunk, and spawns those "rich rocks", in addition to revealing the ore patch, if there is any. If there is none, the rocks should also contain less resources. To have some degree of automation, you could make it so that it requires a wire signal to operate, emits a signal when the job is done, and requires another additional signal to detonate, or alternatively, make it spawn a "buried explosives" tile which requires a wire connection and a signal to detonate, but that feels too cumbersome to set up every time, as opposed to only connecting everything once to the original machine. The machine could require batteries and a mining drill in its recipe, and since it would be a one-time use structure, it should not require electricity. As for automating, a fairly simple blueprint built by a roboport could be used once that's unlocked, and until then, everything can be done by hand.