•When you build something, game remembers that and does not let you build in that place anymore.
(for example you could use cheats to teleport building away, but as long as building is not dead, game will not let you build in that place)
•When you upgraide any building, old building gets removed and replaced with new upgraided building. And while this happens, game forgets that that old 1rst building was build. New building that was spawned on top does not get the same treatment as newly build one and there is no extra mechanism to prevent building
•Usually players can't order to build buildings on existing buildings, but AIs can, sometimes they block their upgraided factories by placing stuff inside factories.
Players can use "repair" hotkey on a building's wreck to give order, even if wreck is on top of other building. however it should not interfere with model of that building... not sure how that works, but you can't just build things inside other thing, but there is lots of empty space around factories where you can build
•you can do some cool weird stuff with that
IT WAS ALL DONE WITHOUT CHEATS, i didn't teleport things inside other things, i actually build it this way, and it can be reapeated with cheats off.
•That is only the beginning tho!
I found out that it is often not consistent.
Sometimes i can fit storage in the corner of the factory, and sometimes i don't
However if all factores are placed next to each other in a grid - then they all are acting similar, in a way that you can build storage in the same corner (i just didn't build all of them on this creenshot, but only 2 storages on the left are buildable for those factories)
•So i have been testing that for long time but im gonna skip all the failed test attempts... here is test that reavealed the mistery
•Making a big grid of dead t1 AAs
•1rst test susbject coordinates 0x0 (mesured in pds with 0 at the top left pd xD), 2 layer of pds are buildable on the left, 1 layer at the bottom
•second subj, coords 0x1, 2 layers at the left, 1 layer at the bottom but only partually... weird actually
•3rd, 0x2, now it gets different, and i am able to build on top 
•4th, 0x3. weird that those two pds were not buildable now 
•5th. 1x0. so changin horisontal position changed my ability to rebuild PDs on factory sides now
After that it was clear that factory's position on invisible grid affects your ability to rebuild things inside of it, witch was kinda werid...
also it repeats itself at least once in avery factory lenght i think... didn't check that for sure, and didn't check if it repeats itself more often... maybe like every 2 1 pgens or so maybe possible.
not many uses for that i guess, can probably try to fit storages and factories next to mexes, but need to know beforehand if you can do that with certain mexes or not. The fact that id depends on position kinda makes things simpler. also makes things insanely more fun for people that would try to recreate what you are doing but not placing things exactly in the same place LOL.
maybe you could fit more shields in the same area and fit tele-defence in the same area at the same time but it requires you to build things and then ctrl+k them and then build on top of them without reclaiming and then upgraide what you have build and then rebuild from wrecks.. not easy, and not cheap
Edit: Testing was hard not only for me https://youtu.be/NhIr38iKFac that poor SACU lost its mind
