https://www.getmonero.org/library/
Ecash/fedimint are a great invention and used under the right circumstances they are a wonderful thing.
The community is about to finish a Nostr client and there is already a bounty to implement ecash on top of Monero.
Because from a pure privacy perfect ecash is a great extension (if also pairs with other privacy tech like i2p) where custody is already given up and is limited in reach and time.
Think gift cards, in conference/festival tokens to buy food and drinks, casinos,....