So I want to make a god-killer campaign and one of my players wants to be a multi-class level 20 wizard and level 20 fighter. How would I go about hp and pro bonus? I was thinking of just continuing from the level 20 wizard and increase the pro bonus as regular for the next batch as a fighter and how would I do HP. Would I continue HP increasing as a fighter or should I still keep going as the base model which was the wizard and on top of all that, I wasn't sure if increasing the proficiency bonus of what the guy knows as a wizard to what he knows is a fighter would have all been the same or is it to separate charts. He'd be a wizard level 20/fighter level 1 with a pro bonus with a greatsword of +6 seems weird to me.

Up is based on hit dice. You get hit dice for each level of a class. Different classes have different hit dice. A level 9 Wizard/1 fighter would have 9d6+1d10 hit dice.

Characters only have 1 proficiency bonus, it is based on character level.

Maybe check out these rules: https://www.dndbeyond.com/compendium/rules/basic-rules/customization-options#Multiclassing

Thanks