Right, this. Plus, the paper was probably written by teams, in sections, and giving aggregated feedback.

Rather than attempt to disentangle, large physics collaborations normally have a “You are on all papers we publish while here and X years after you leave”.

Yes, that's basically how it works (kind of like movie credits). On papers with fewer authors, there's a kind of ranking system in the order of the authors: the first author is generally the one who did the experimental work and the actual write-up, the next few may have helped with the write-up and run some of the experiments, the next may have just helped build some of the experimental setup, or otherwise had some material contribution to the research, and the last author is usually their supervisor who reviewed the paper before submission. When the list grows really large like this they usually give up at some point with trying to rank the contributions and just list the rest alphabetically.