I can plan your cross domain setup, but I need a few things first so I don't configure the wrong domains.
- Domains: every domain in the journey from your site to the checkout and back, for example [your site domain] and [your checkout domain], in order.
- Hops: how a visitor moves from one domain to the next (a link, a button or a form that posts), and whether any step redirects. Redirects can strip the parameter that carries the visit across domains, so each one needs a test.
- Tag info: which of those domains have the same GA4 measurement ID, and which you cannot tag (for example a hosted payment page). Only domains you own and want to include get configured; a domain you can't tag is handled as an unwanted referral candidate instead.
- Referral rows: the referral source names and session counts you see for the checkout, copied as they appear in GA4.
Also tell me whether you have Editor or above on the Google Analytics account, since the setup needs it.
Nothing in your GA4 property changes until you approve the plan.