Checking account "Bill Pay" features are a fusion of multiple ideas that you access through a single interface.

Adding a biller to it involves typing in the biller name, providing the account number (e.g. your credit card number) and the ZIP where your bills say to mail a check.

Big billers (like say, Chase credit cards) register with the other big banks and the handful of companies who provide white-labeled platforms for all the small banks and credit unions so that when you input one of their accounts this way, it'll match it up and turn into an electronic transfer which tends to show up pretty quickly (though I assume it does still take a couple of days to fully clear, you'll usually "get credit for it" the day of the payment).

But you can also just add literally any person or company name and address. Say, Bob Smith, 123 Example Street, City, ST 12345 and they'll just mail Bob a check on your behalf. So, those will always require days of time because they have to print and mail a physical check in those cases.

So, the generic advice has to assume it's not matched to an electronic biller profile.

In the case of most banks I've used, they hide this implementation detail slightly -- they just always have you pick the date the payment 'will be received' - and when it's not an electronic biller, you just can't pick a date that's too close.