How do I get LEGION Tech?
First you need the schematics. It is possible to find them in loot crates, but the most surefire way of turning your enemy's tech against them is by raiding LEGION Lairs. Unfortunately, all the lairs you'll raid as part of the main story and character missions don't count- you only get blueprints from stand-alone lairs in Seoul, or lairs uncovered in Global Conflict.
Running the lair itself is a random experience- you might get a two room dungeon or a ten room, you might face three poison gas chambers in a row, or find a lair consisting of unguarded treasure rooms. In any case, the lair's contents have no correlation at all to the Legion Tech earned. Once you exit the lair, you'll get three LEGION Tech schematics, one for each squadmate. As with gadgets, squadmates that have already collected all their LEGION Tech will earn a blueprint for a random MAYHEM agent that's not full up.