The vendor has custom modules for auto-updating. afaik F-Droid also auto-updates on freer distributions of Android but not on most manufacturer ones.
Exactly. This is because F-Droid requires root access to provide this functionality. I'm not sure if the module for this is still up to date, but it is a Magisk module that could be installed on any Android device rooted with Magisk.
Well I actually also have a Note 9 and it seems Samsung stuff generally does not get updated by itself & either stays outdated or nags for update I need to manually trigger.
Stores that are baked into vendor images are granted special privileges to act as app stores.