There are two ways of zapping:

1. Zap from your account by first depositing some sats into the provided LN Address
2. Zap from your own Lightning wallet by long-pressing the bolt icon and choosing the zap amount

Uhm, that's wrong 👀 Choosing a custom zap amount does not change the wallet. Not sure where they got this from?

To change where SN takes the sats from, you need to attach a wallet here.