Containment units are optional areas that offer some helpful upgrades to your organism. For the most part they are passive enhancements. You will need to have all the main abilities in the game and have accessed every area before you can get to each containment unit.
https://gameplay.tips/guides/8163-carrion.htmlmap by demajen
The above map will be incredibly useful to find your way back to each area after having made it to the final area. The video will show you how to get to each containment unit from where you exited after completing the area, though a couple can be found on the way through the first time.
00:00 Intro
00:47 Military Junkyard
02:16 Armored Warfare Facility
02:55 Bunker
03:43 Frontier-Wastelands
04:30 Control Room
05:00 Nuclear Power Plant
07:11 Leviathan Reef Base
08:28 Botanical Gardens
10:44 Uranium Mines
12:23 Relith Science HQ