Select "Communities" up top, then filter by "All" (not "Subscribed" or "Local"). Either browse the list for Lemmy communities, or search for words related to communities you want to join. The results displayed from ALL Lemmy servers that your current server is connected to (federated with).
https://browse.feddit.de/ - A Lemmy community browser. Not sure how or if it achieves anything different from the above, but worth a try as well.