There are four Weapon Upgrade spots in the Avalon map. The best thing about the upgrade is that if someone in your team finds it, the location will become visible to all the other players in the team. They are not all tied up and unlocked behind any tier system; you can access the Legendary Workbench from the start without having to climb the ladder. Here is the location for all four Weapon Upgrade Locations in Call of Duty: Black Ops 7 Endgame.
All Workbench Locations in Avalon

The picture contains all the Workbench locations on the Avalon map.
#1. Uncommon Workbench | Car Factory
It’s located in the Car Factory, on the southeast side of the map. Now head to the building on the northeast side of the area. And you will find the Uncommon Workbench inside.
#2. Rare Workbench | Listening Post
Now you need to go to the southwest side of the map, into the region Listening Post. You will find the Rare Workbench in the storage section of the server room.
#3. Epic Workbench | Resorts
Head to the north of the map to get to the area named Resorts, and then get to the small building located west of the Resorts area. When you get to this area, you will see the sign on the entrance that reads Police. Get inside, turn left, and then jump over the broken window on the right to find the Epic Workbench.
#4. Legendary Workbench| Fort

This one will take us to the Fort area located southwest of the map. Once you are in the spot, head north
#5. Legendary Weapon Workbench II | Low Town
This can be found in the southwest of the Low Town area. You will not find the Workbench on the bottom floor; take the stairs to the upper floor to find it in the office.
If you join a team, and the teammate has already unlocked the Legendary Weapon Workbench weapon in the game, then you can also upgrade your gun to the Legendary status. But be wary, when you are stepping into the Legendary Weapon Workbench area, the enemies around will be tougher and will hit hard in Call of Duty: Black Ops 7.

