The issue:

I initially kept seeing another web site when linking to Google Analytics in JSitemap.

BUT I couldn't find the UA code in my new Google Analytics GA4

So I deleted the account and started again.

The important thing to do is in Step 2 is to click on Show advanced options

When you click on show advanced options you get the following screen:

Move the button to Create a Universal Analytics Property. The GA4 measurement is mainly for those sites that have an App associated with it. Create a Universal Analytics property only should be sufficient. Once created you can then find your UA analaytics code.