This is an overview of all sites where you can find the chests containing the weapon modules. Once you collected all, be sure to un-equip all modules from the weapons; they need to lie loosely in your inventory to obtain the achievement.

Mazum (found in the south, close to the Sodoth Shore Daoka), an old friend known from the prequel. helps us with creating weapon modules when we bring him module parts, scattered around Adelpha. You don't need to find all, as indicated in the boxes above.

I found this guide, and I thought I rather link it, than stealing the image and re-uploading it here.

All module parts locations[estnn.com]

Inside Desan, on the left, at the main gate, close to the daoka bridge, are shelves with "collectible" signs. They will be filled after having started the rebels raids (main quest). During the play they will steal three small container chests, which eventually can be found in stolen containers in the cupboard on the left. This takes a while, until all containers appear.

p.s.: This probably has been fixed, but there were occasions where upgrades were either all piled into one container, or there were no containers visible at all - all upgrades where still collectible on "use".