at protocol & nostr are quite different both in architecture and aspiration
The best thing to do is go through the documentation
Read the overviews here
NOSTR:
https://github.com/nostr-protocol/nostr
AT PROTO:
https://atproto.com/guides/overview
And then I would recommend comparing the first 5 implication possibilities (NIPs) on nostr:
https://github.com/nostr-protocol/nips
To the identity / repo / lexicon / app sections of the docs on at proto: