I have sent 50 dollars to a card that’s not on my phone and I don’t know how to get it back

If that bank account belongs to you, you would access the funds in the same way that you do with anything else in that account. If it was an error and is not your account, see the following from Transfer money in Apple Cash to your bank account or debit card - Apple Support

If you think that you entered the wrong bank account or routing number, contact us.

You probably have transferred it to your bank account.