How to get all blueprints?

Trying to get all blueprints (not outfits or heads). Besides using the blueprint extractor what’s the best way to do it? I also heard that once u unlock more Boss Stem cells u should use them bc more enemies spawn/ drop more blueprints. So should I always be on highest difficulty. Also does custom mode or assist mode affect getting blue prints? Would it be easiest to just turn in assist and custom mode and run through the game getting shit? Probably won’t do that yet and just keep playing and progressing but if that works I will try it someday lmk??