why not other countries?

manxman111 wrote:

I have UK Halifax bank account in my iphone wallet, but how do I apply for an apple card?

You cannot. Apple Card is only available to US citizens and legal residents at this time. Link-> How to apply for Apple Card - Apple Support

The apple web site states it's for the US only.

Correct.

Is Apple card a credit card?

Yes.

Is it different to apple pay?

Apple Pay is the the overarching mechanism that lets you use payment methods like credit cards and debit cards with your iPhone in stores and merchants. Apple Card is just another type of credit card you can use with Apple Pay in the same way you use it other credit cards and debit cards.

Link-> Countries and regions that support Apple Pay - Apple Support

Apple Pay is available in countries where Merchants and banks have chosen to support it. Apple cannot force them to support it.