Fuga: Melodies of Steel 2

This achievement refers to the items you can upgrade in the workshop: weapons, armor, and reactor. You need to expand the workshop to lvl 5 first to unlock the ability to upgrade anything to lvl 20 (max level). For lvl 16-20 upgrades, you'll need ancient-level materials or the equivalent of. Except for titanium, they are not found in shops ever. You can get them by:

• all materials except refined crystals found in lvl 5 difficulty scrap fishing, unlocked by expanding fishing stand to lvl 5
• refined crystals found in Shrine of the Moon God exploration
• chance for all materials in late-game or NG+ map pickups
• low chance for all materials as late-game or NG+ battle rewards, higher chance against elite enemies

The earliest for all of these are in late-game, so you'll probably be getting this in NG+.