The random characters are because Nostr uses crypto (as in encryption, it's also where crypto currencies get their name but nostr is not running on crypto currencies) keys to identify accounts. It looks a little less weird on the Nostr side, but the fediverse & Nostr are bridged and sometimes it looks a bit weird when that happens. (Shoutout to Mostr.pub which makes it possible).

The difference between decentralized and federated is Nostr accounts are not tied to a specific server but created locally and mirrored on relays (servers), and Activity Pub is set up on a particular server that can communicate with other servers. Pros and cons to both.

Even if it looks a bit weird, they're probably just following you because you post good content.