So I can’t really use this card at supermarkets or Walmart?

Your question was answered.

Apple credit card is usable anywhere credit cards are accepted.

If you mean Apple pay, you can only use it where it is accepted.

Walmart doesn't accept it because they get no tracking information from it.

I suspect that the grocery stores that don't allow it are doing so for the same reason.

The Apple Card uses the MasterCard payment system for processing transactions, so it can be used anywhere on the planet where MasterCard is accepted.

The use of the NFC Apple Pay service is restricted to those companies and web sites that allow it - Apple Pay - Where to Use - Apple. Apple Pay is also not specific to the Apple Card - you can use whatever credit card you want with Apple Pay.

Anywhere that credit cards are accepted.