I’m no expert and can’t answer most of the questions, but as far as the API, I believe it is based on REST
I believe that ? part is the method or command or operation to be requested/performed, and & are ancillary options or parameters for the requested operation.
as far as the session history, I believe that is YOUR history, not the user you are referring to.
you might check out the web app TAUTULLI, the code there may better inform you on what api calls can do/can provide for you.
this post @ Big Media folder? Make smaller video preview thumbnails! gives an example of using the api via command line, and a link to a simple way how to get a token.
see also @ Authenticating with Plex
if you just want an easy way to see history within a browser, just go to @ Plex Web > settings > dashboard > scroll down to history section (this may require plex pass I don’t know)
full history will get a filterable list of all history
