Total ATK calculation method:
ATK = [( ATKCharacter + ATKWeapon) × ( 1 + ATKBonus% )] + ATKFlat Bonus
Where:
ATK is your character's final attack value, which can be viewed from the character's Attributes screen
ATKCharacter is the flat attack value your character receive from levels and ascensions
ATKWeapon is your weapon's flat attack value
ATKBonus% is the percentage bonus you get from all sources, that's specifically listed as ATK % (not damage %). This includes buffs, artifacts, weapon passives, skills, etc.
ATKFlat Bonus is the flat ATK you get from artifacts, skills, food, etc.