Overview
A custom product page is an alternate App Store page for a specific audience: its own screenshots and promotional text, on its own link. A product page test compares versions of your page against each other.
Both live under Experiments, on two tabs: Custom product pages and Product page optimization.
Eligibility
Custom product pages
Create starts a page. Give it screenshots and promotional text for the audience it is aimed at, then submit it the way you would a listing change.
A page has its own review state, so what you can edit depends on where it is in that lifecycle.
Product page optimization
Create test sets up a comparison between treatments of your product page. Apple runs the test and reports the result in App Store Connect.
Common questions
I cannot see the create button.
Apple has not reported your app as eligible. The button is hidden rather than shown-and-broken, which is why the tab can look empty.
Can the agent draft these?
Yes, with the Experiments scope granted, and only on an eligible iOS app. Its drafts arrive in the review inbox like any other proposal.