They are a green item that goes in your inventory, same place that planet resources and tokens and stuff go. You can only get them on mars and can only hold 4 at a time, which you combine into the blue thing which opens one specific node on mars. You can only hold 1 of the blue item at a time. The nodes have never been shown on the map, you need to find those by the description on the blue item. (Or use a guide) I’m at 8 so far myself. Sadly the ones I opened for a different quest didn’t count towards it :( Escalation protocol chests, heroic events, and high value targets, as well as payrolls (all of those only on mars) all have a chance to give the stems. I think you can get them as random drops too but I’m not sure.