Cashu has one bitcoin wallet in charge of the lightning node.

Fedimint has a lightning node requiring multiple wallet keys (multisig) to withdraw onchain funds.

The ecash system is largely the same betweem fedi and cashu.

They are both mints. They both "mint" redemption certificates (chaumian blind signature ecash). Neither can provide a cryptographic guarantee that there is actually bitcoin "on reserve" at the mint, or that your ecash is backed by actual bitcoin. This is why fedimints exist, so it takes a concensus of custodians to drain the actual bitcoin the certificates represent.

Ecash certificates are only redeemable at the mint where they were issued. Think of it as a money order that's only good at the bank that issued it.

If the mint's lightning node doesen't have lightning channels open, or outbound lightning liquidity, then the only way you can redeem your ecash certificates for bitcoin (if the "bill" is on someone else's mint) is to ask the node operator to send onchain bitcoin to your personal bitcoin wallet address.

The only other way to use the ecash is to send them to someone else that has an "account" at the mint where the ecash was issued.