Chel - Get to capital city, then go back to the scrapyard, talk to the guy there and he will say "bring me 3 mega scrap for the upgrade", those seemingly can appear on any map, but the upside is that if you go to a map that has one it will make a ping sound to notify you to be extra on the look out. Got this one aleady. (credit for this goes to Palex00, who posted the original Chel Mega scrap info, thanks for that!)

Darris - Get to capital city, then return to silvington base, a scientist in the entryway throws a hissy fit and gives you his key, use this on a door in the room where you fought Cherk for the last time, fight boss in there and get Robo Dudes upgrade, Got that one.

Tanstrom - Get to capital city and progress the story until you are in the sewers, once you have the boat head to the left of the map, down a tunnel will be a room with a computer, one of the options on this computer will unlock the bunker located at Hogtowne Checkpoint, Tanstroms final upgrade is inside. (credit for this goes to Neon_Phantom in the discussion post, Thanks!)

Penton - Penton's final weapons is found the in the Biodome Spaceship - Forest area, it will be on the leftside in a building a little ways in. Once you have this there are no schemas or upgrades past this point, so if you are missing one it will be somewhere further back in the game.

Final Schema- Nuke- This is located in a bunker just to the south of the Kovel travel point, I believe it was called the Alpha bunker or something like that. There is a laser barrier that blocks entry, to clear the laser barrier Get to the capital city and progress the story, past the point that you have to destroy a wall there will be a computer terminal that will unlock the Kovel bunker. Once you use it the laser barrier will be gone. *WARNING* I dont know if it is all builds of the game or just mine, i was playing on Steam Deck, but EVERY enemy fight started in the Alpha bunker will cause your game to crash. Thankfully the game saves between floors, but reloading is likely to be necessary at this location, I had to reload 20-30 times but it IS possible to get to the end, get the schema, and get out without having any fights.