What happens when you don't evolve Pokemon?

Nothing. The pokèmon doesn't evolved and you can train it normally. Remember only that every lvl after its evolving-lvl, it will try to evolve, so if you wanna stop it definitely give it an everstone

(a pkmn not fully evolved at lvl100 never can evolve to the other stages)

What you said is true but let me just point out that the smart thing to do is stop your Pokemon from evolving until it is advancing to lvl 100 because when you are in early stages you gain levels and learn moves faster

The lvl 100 no evolve exeption is stone pokemon like Jigglypuff. They will not learn anything sadly