Primer is a very complex movie and hard to impossible-to-impossible to understand in the first watching. This is due to the fact, that we only see a small part of the actual happenings in the story, and the scenes are not in a chronicallychronologically correct order.
This is hard to spot, as the scenes seem to fit after each other, but sometimes they are reruns of a scene that is replayed by time travelling older versions of the characters, thus resulting in strange behaviour or dialogues.
One thing to look out for is the earpiece Aaron is wearing in some of the scenes. The Aaron from the future has recorded all earlier conversations, travelled back in time, replaced his earlier self and tries to recreate the dialogues this way, sometimes failing (e.g. in the basketball scene, where he misses the throw). So every time you see an earpiece, you know that this scene is not chronically in chronological order.
I would recommend to readreading this very good essay, studystudying the timeline posted in OddCore's answer, and then rewatchrewatching the movie once or twice. It helped me understand the movie to a point where I can more or less tell what is going on.