It's documented here:
By default users on an instance will be able to talk with communities/users on all other instances. This only changes if the instance admin puts hosts in the allowed
list or disables federation.
If you add instances to the blocked
list then users will be able to talk with all other instances, except those on the blocked
list
I think the description would be too long and clutter the table. I'd be down for descriptions on-hover, but I'd have to switch platforms (from GitHub markdown) for that afaik.
You can also get the country from this list. I don't know how they do it (maybe IP lookup)