Your receiving wallet on nostr is defined in your user profile settings. Any zap you receive from anyone on any nostr app will go to the wallet defined in your profile. Your sending wallet depends on the individual app you are using at the time you send it. Some wallets like Damus give you a wallet selector when you send a zap, or even allow you copy the invoice which you can paste into any wallet to pay it. Other wallets are more limited in their options for sending. For example, the Primal iOS app can only use the built in Primal wallet for sending, but their Android app can use either the built in wallet or NWC. The Primal web app on the other hand can use the Alby browser extension for sending. So sending is different depending on the client/app. Receiving is always according to whatever wallet is in your profile settings. Hope that makes sense the way I explained it.