I feel the game is pretty easy to learn. There are some things that are communicated poorly though, I do agree on that. But for example, abyss D is just an endless abyss mode that gives you benefits based on your further stage.. you can find this out by literally just reading it, it says you get smtg at stage 20. Then when you reach stage 20 it reveals the buff & reveals the next stage requirement.
You can find out more info in the info section about corruption as well as just hovering over it below your level iirc. The main most important part is that it reduces your max level above level 300.
Mutagens/radiation/celestials were all explained pretty okay in the info section of the game. Celestials have a chance to spawn based on your danger value, you can gain mutagens from enemies above certain stage levels, higher tier mutagens are more valuable. Radiation is just spend mutagens into perks, ez pz.
Notice when you make a skill infinity in the tree, it says it costs P and not IP. You're still spending normal **P**oints, ya know.. the same points for the rest of the tree?