All cards calculate interest per day. That’s the standard formula. They only charge you the sum interest at the end of the month when your billing cycle ends.

https://www.nerdwallet.com/article/credit-cards/how-is-credit-card-interest-calculated

This becomes apparent if you’ve ever paid a card off really early, and then still been charged interest even though you’ve had a zero balance one or two weeks before the billing cycle ended.

https://www.bankrate.com/banking/savings/best-high-yield-interests-savings-accounts/

This was the first result that I searched. You can get a HYS with a little to no minimal balance. A lot of them are online banks, but they’re still FDIC insured. There are plenty HYS accounts out there that are usable for everyday people given you have the ability to save.