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 it has & each guardian's status, the network it's running on, the number of confirmations required for on-chain transactions, and more.