By reading others questions like Highest damage in one melee attack and Highest AC possible.
How many hit points can a character have, and also highest achievable hit points possible in an instant?
We’re looking at a level 20 character and no multiclass.
Feats are allowed.
Magic items are those on the tables in the D&D 5th Edition DMG.
Spells, skills, feats and abilities available to a player character are eligible for use/consideration.
No other help from an ally.