I found a weapon blueprint and invested cells at the collector until it became "unlocked". A copy of the weapon dropped just then - great! But how can I access that weapon in my next run?
After you have unlocked the weapon by spending 10 cells, and getting it right there, it starts to spawn randomly in the dungeon. It can also be a starting weapon, if you have unlocked the randomization for those. As quite a lot in this game, and typical for a rogue-lite, much of the game is randomised.