What if someone sent money off my phone to a scammer

Depends on whether you can contact your bank or your credit card company in time to void the transaction. The scammer will still have your bank, credit card info, etc. so you will need to close that account immediately.

Not sure how someone else could send money from your phone unless they knew the passcode for your phone and log on info, etc. for the financial institution.

If "someone" sent money from Apple Cash on your phone to a scammer, the money is gone. Apple Cash is just like using cash.

In the future, don't let "someone" use your phone.

Then they probably got the money. What is your actual question? How is it that "someone" send the money from your phone?