Your description sounds like things that can already be done in Vanilla. Construction in space and dropping items from orbit are already available. What did you change?

Yes, the technology is now unlocked way earlier to support taking planets.

I am still confused, what does the mod actually do?

Is there supposed to be a blueprint to go with this? or is this just a base platform we're expected to fill in with machines?

I was super interested in what this did too, I just ran some quick tests and this is what I found.

Motherbase Platform
The "motherbase" platform is automatically added to the save when the mod is added, it is created at the start in new saves, and is also immediately added upon adding the mod to an existing save, so it works either way.

The spawning "motherbase" includes the Platforms, Solar Panels, Accumulators, Walls, Asteroid Collectors, and Cargo Bays that you can see in the Information preview (All rare quality). A quick CTRL+C scan of the platform reveals it has 5.8k Space Platforms, 546 Walls, 123 Solar Panels, 90 Accumulators, 22 Asteroid Collectors, 8 Cargo Bays, and the Platform Hub of course. So those are all the items you are immediately receiving for free upon downloading the mod.

Technology
Moved Tech: Advanced Asteroid Processing
Prerequisite: Space Science
Requirements: Craft 1 Crusher
Effects:
Advanced metallic/varbonic/oxide asteroid crushing, Advanced thruster fuel, Advanced thruster oxidizer
Notes: Previously Post-gleba, Now unlockable almost immediately after getting space science. Unlocking this much earlier feeds into the other techs that are allowed earlier, like now being able to create coal on the platform when combined with the next tech.

New Tech: "Coal Synthesis"
Prerequisite: Advanced asteroid processing
Requirement: Craft 50 Carbon
Effects:
Ice melting, Coal Synthesis (recipe) Ingredients: 1x Sulfur, 5x Carbon, 10x Water, 2s. Products: 1x Coal. Made in Chemical plant.

New Tech: "Simple Coal Liquefaction"
Prerequisite: Coal Synthesis
Requirements: Craft 30 Coal
Effects:
Simple coal liquefaction (Oil Refinery recipe), 10 coal, 2 calcite, 25 sulfuric acid, 5s. Products: 50x heavy oil. Made in Chemical plant

New Tech: "Cargo Controller"
Prerequisite: Space Science
Requirements: 1000x Red, Green, Blue, Purple, Yellow science. 60s
Effects:
Cargo Controller (Recipe) Ingredients: 10x Rocket fuel, 10x Steel chest, 10x Logistic robot, 1x Roboport, 5s. Products: 1x Cargo Controller. Made on Space Platforms

New Tech: "Depleted Uranium Ammo"
Prerequisite: Nuclear Power + Purple Science
Requirements: 1000x Red, Green, Gray, Blue, Yellow science. 45s
Effects:
30 x Depleted Uranium Round Magazine (Recipe) Ingredients: 1x Depleted uranium fuel cell, 30 x Piercing rounds magazine, 30s. Products: 30 x Uranium Rounds Magazine
Description: You paid[ed?] for the fuel cell, use the whole fuel cell.

So it definitely adds more than just things that are already available in vanilla, but the only thing I'm unsure about is the "Cargo Controller" as it acts like its supposed to be a new item to send requests down to planets, but as far as I'm aware this is something we could do in vanilla. It looks like a logistics request screen on a roboport up on the platform, but we could already request things to planets from platforms by creating the request on the landing pad itself I thought? So I might be wrong about that, but personally I think this mod does add a ton of cool stuff (like moving the techs around to get advanced processing earlier, and making a use out of depleted cells, but the whole free platform seems like a bit much to me personally as the mod currently stands without any config options, but the mod does say its in beta so I'm not gonna be overly critical

tldr; instead of needing to wait until Gleba for Advanced Asteroid Processing, you instead unlock it immediately after unlocking a new "coal synthesis" tech, that is unlockable immediately upon creating your first space platform. This means you are able to immediately do advanced asteroid processing, and create coal on the platform. Next, it adds a "Simple Coal Liquefaction" tech, which means you are able to create heavy oil on your platform out of that coal. There is also a "cargo controller" which is meant to send items from the platform down to planets, but I think that may be possible by making requests from the landing pad already. There is also a random extra uranium ammo recipe for some reason (its a cool idea, I just don't see how it fits this mod). Moving away from tech, you do get a free space platform with a decent chunk of rare Solar Panels, Accumulators, Asteroid Collectors etc. As big as the one seen in the preview

Edit: Formatting

Edit 2: The version of the mod while I ran these tests was 1.0.2

Hey lulunexi, Thank you for the detailed review!

With regard to the Cargo Controller, its designed to send items to other platforms near by, with the use case of creating new space platforms, we had to mod this as the game has no option to transfer items directly between platforms, without paying the launch cost twice or building a seed ship that picks the hundreds of space platforms from Vulcanus and fly's back to safety to construct.

Functionally its a chest with a GUI, giving the option to send any items to other near by platforms
(this is scripted as the game has limits around surfaces/platform interaction)

Also if there's a better description we can add for the cargo controller, would be happy to get feedback.