I don’t know really, as I haven’t explored too deeply, but logic dictates ‘Guest’ would just be a Home User without a Name, Account or Password that gets their own watch status and shares as if they were a regular account holder. Guest would be ‘The Spare Bedroom’ so to speak and anyone in it would enjoy it’s benefits.
If it works differently from that - logic doesn’t enter into it… apparently…lol
The Guest user is a ‘managed user’ with a specialty.
It is mainly intended for a device which is under the control of the host, i.e. you.
It is not very suitable for BYOD.
You sign-in the device with your own Plex credentials and then ‘Switch User’ immediately to the Guest account.
Which means that all the other users in your Plex home need to have a PIN set, to avoid the guest from switching user again.
When the guest is gone, you disable the guest account. Which resets all ‘watched’ status, ready for the next house guest.