Any instance can connect to any other instance by design. That includes any of the communities (sub Reddits). Any community is available from any instance.

So if you join a general instance (like reddthat.com #selfplug) you can search in the communities and find any other community and subscribe to it.

browse.feddit.de is the current place that searches every fediverse instance, and indexes all the communities. So your can find any community, even if the instance has not federated yet