Once you have completed the story for the first time on any difficulty you will unlock the Extra Content Shop. You can access this shop through the main menu under Bonuses. Character and weapon models are purchased through this shop.
The Extra Content Shop uses a type of currency called CP (Completion Points). CP is earned by completing challenges throughout the game (many of these challenges are the requirements of achievements).
A list of all the challenges and the CP rewarded for their completion can be found under the Challenges menu under Bonuses or can be found here:
https://residentevil.fandom.com/wiki/ChallengesThere is a total of 76 character and weapon models that are required to be purchased for a total of 8,900 CP. You are awarded 1,000 CP for purchasing all 76 so you will lose 7,900 CP total.
After purchasing all 76, you need to go into the Figures menu and scroll through all of them so that the red notification is removed in order to unlock the achievement. Viewing each individually is not required.