Let's say I have some Sats on Zap Desktop wallet. Is it possible to transfer the Sats out of the Zap Desktop Wallet to for example MyCelium?
Yes. You always have two accounts on LND (ZAP interfaces over rpc with your self hosted lnd node). One account is your lightning wallet where channel management is done, and the other is your on chain wallet. Depending on where your balance is there are different processes for moving the balance to your on chain wallet (which is what I assume you want).
- Moving balance from lightning channels - You need to close the channels on the ZAP wallet which will broadcast the closing transaction to the bitcoin network. Depending on how long the HTLC is, the balance would take some time for it to become spendable.
- Moving on chain balance - You can simply transfer on chain funds to your other wallet just like any other on chain transaction.