Char ATK + Wpn ATK  * (1 + ATK%) + Flat ATK
Char ATK = Base ATK without counting any gears. This is increased per level

Wpn ATK = Just the Main ATK for your Weapon

ATK% = All your artifact ATK%, Wpn ATK% buff, ele resonance (Pyro), other %ATK buff like food buff

Flat ATK = Feather Main stats + artifact ATK, flat ATK buffs

Edit: For your example, its possible that it is included with food buff + other buffs