my question about the penetration in general

what happens when you decrease your opponent's armor to - values?

lets say enemy has 50 armor and my armor pen runes,masterieses, youmuu's ghostblade,lw can delete his armor and theoricly he has -15 armor now. Would my damage still be same as he has 0 armor or would it be even more?

LW and % penetration works first or flat penetration decreases first?

Can someone really explain me the penetration rules? Because I sometimes see pro players who build way more armor pen. than they needed instead of going damage, and I really don't know how that works.

Thank you guys.

edit : I saw a challanger zed stream, enemy team were full of squishies who had 0 armor item at general but he was insist of building armor pen. He has arpen runes, rushed youmuu's, drakthaar,BC,Last whisper. If i were him I would go AD after Youmuu's. Since I am noob about this topic, I want to learn about it if you guys can help me.

The reason he wanted to have that much Armor pen. was having more damage or, since he can 1 shot anyone at this point he just wanted to gain other stuff from armor pen items such as CD etc, or he wanted to build to 1 shot safely in case if they go base and start building armor etc.???