Don't know much about SUPL other than what the website says about it:
If your chosen carrier includes the supl APN type in their APN configuration, SUPL will be used to provide A-GNSS in order to greatly improve location lock time for GNSS (GPS, GLONASS, etc.). The fallback SUPL server is supl.google.com if the carrier doesn't choose a specific one. You can remove supl from APN types if you don't want to use this, but it will greatly increase GNSS location lock time if your carrier lacks control plane A-GNSS via the cellular network and fully relies on user plane A-GNSS (SUPL) to provide this instead.
dortlanders If enabled like default it connects to supl.google.com, what does this do?
I don't know the answer to this one... However, it looks like they're working on setting up a SUPL proxy so you'd be using something like
supl.grapheneos.org by default instead of Google's (source).