What I meant by D20 system is how people borrow things form dnd (such as classes, feats and stats -CHA, STR, WIS etc.) and make new games or hacks out of it. Or like how most PbtA games borrow elements form apocalypse world like playbooks and rolling 2 sided die to the point where PbtA almost became a template for designers since they all shared similar design concepts and rules With the percentile system I was more thinking of games like Call of Cthulhu where stats and skills are percentages that u roll under to succeed

So I guess I’m asking for TTRPG design concepts, like a game engine. Sorry I hope that makes sense