Behind the scenes the money doesn't actually move day of. The people that accept payments are doing you a favor and crediting you before the funds settle.

If you pay a bill on the 1st, the money does not land in the payees account that day (barring in person cash transactions). Best case scenario, funds are available on the 2nd to the payee, although, more commonly, it is several days before the funds are settled. Banks will frequently paper over this showing the funds where they will eventually go, but the clearing house still hasn't finished. If you try to spend too much of that unsettled money, the bank will be glad to inform you that the money isn't actually available yet.

When people talk about instant payments they mean a few seconds, not overnight -maybe.

I used to be in charge of a payments dev team that oversaw several billion dollars of bill payments per year. There is nothing instant about moving money in the US.