Where can I learn more about specific federations?

Fedimint Observer is a helpful site that contains additional information on publicly known federations. The Home tab shows a list of all observed federations, along with their public ratings, invite codes, total assets, and average activity over the previous 7 days (number of transactions & total volume).

Clicking into a specific federation takes you to that federation's page, where you can see how many guardians are running it along with each guardian's status, the network the federation is running on, the number of confirmations required for on-chain transactions, and more.