Is it posible to add "death count" like in turrets, and based on this increase fire power, hp, shield or auto repair with high level/"death count".

it is getting more possible, now that the developers have added the 'killed by' entity reference in the on_death event. I can now see a biter's death, and see if the entity is a soldier unit, and then give that soldier (or his squad) 'experience points' and try to make them better or different somehow. But that is a lot of work, and the 'unit' type I am using in the mod doesn't have much I can modify, unlike player characters who can get cool things like increased movement speed or crafting speed etc.

the low tier robot can have up to 5 levels, the lat TERMINATOR can have more, depend on how many level You pla, change can be collour, size or stars (like army bars) every feve levels like 5 or so, after adding "killed by" there could be function moddfiedd from: "create_follower_upgrade" from base facorio tech, or like from mod: https://mods.factorio.com/mods/dewast/Tech_add,

Well yeah, the 'levels' is arbitrary, but you cannot modify the prototypes once the game has started and the "Unit" entity type has NO modifiers or easy upgrade skills to make use of. I could have a script running for each squad that heals them faster based on their level, but that's it.. and will be bad for performance.

What about ammo type damage or in tech like for turrets, or ammo for gun independent of levleing up in tech tree.
The next "level" of this mod or mod modyfying this one (so people can chose) could be to make TERMINATOR frame and all the rest add in grid like for vehicle: shield, generator, bateries, and gun: machine gun, flame trower, cannon, laser or rocket even support type - with roboport and robots to heal with repair toools. For people with hard mode it can have 4x4 grid and equipment 2x2 - so 1 shield 2x2, 1 batery 2x2, 1 generator 2x2 and 1 gun or roboport 2x2. And of course tech MK for all grid equipment to get stronger and stronger but more expensive unit.

I can't make a grid on a unit - i'd love to, but the devs wont allow it for now (or ever). I have the same ideas as you.

As for ammo type damage and tech, "level up" cannot change the ammo type, because that must be set/changed only when the game starts. And the damage tech affects all robots, not just the squads or single soldiers who have 'levelled up'. :(

Is there any chance to use "killed by" count, and by "devs" You mean core program? If so, do You know how was Vehicle grid added, by the game or by player mod?

the vehicle grid was added to the game by the developers, in the core program. Modders were able to add grids to all vehicles after that. But "units" are still not able to have grids. We can use the "killed by" counter, yes - either per-unit, or per-squad. But there is no easy or useful way to give the squad new abilities or increased stats based on kill count, because the core program does not allow this to change for this type of unit. Only "character" unit has lots of easy abilities to change.