how do i transfer funds from apple cash to the apple savings account?

Set up and use Savings in Apple Wallet - Apple Support

Deposit money into Savings

You can add money to Savings from a linked external bank account or from Apple Cash. 

1. On your iPhone, open the Wallet app and tap Apple Card.
2. Tap Savings account, then tap Add Money.
3. Enter the amount that you want to add, then tap Add.
4. Select or add the payment source that you want to add money from. (Select Apple Cash here)
5. Double-click the side button to confirm with Face ID, Touch ID, or your passcode.

Money that you deposit into Savings starts earning interest on the day that the transfer is initiated from the Wallet app.

If you transfer money from Apple Cash, the funds are typically available instantly. If you transfer money from a linked external bank account, it will generally take a few days for funds to be available for withdrawal.

Once you set the amount, and press next, it should show you the default account that its has if you press on the right arrow there, it should list your available sources, with Apple Cash at the very top: