Care to point to the documentation? AFAIK there are 0 open source server ("store") implementations compatible with recent versions of snapd.
I wrote it up here.
https://www.theregister.com/2023/11/10/snap_without_ubuntu_t...
There are multiple links to Canonical docs, and I don't think it's worth trying to summarise it further.
I found this... Not really "Canonical docs", no clear way to support multiple architectures, and it hasn't been updated in a couple years, but I guess it works...
No it isn't. Canonical's snap store is closed source but Snap does not depend on it and the process of configuring snapd to fetch and install from your own store is fully documented and needs no proprietary or Canonical tech whatsoever.