Cross-domain measurement allows you to see sessions from two related sites (such as an ecommerce site and a separate shopping cart site) as a single session, rather than as two separate ones.
Configure your domains via the Admin
For Google Analytics 4 properties, configure your domains via the Admin web interface to automatically enable cross-domain measurement.
Domain configuration in the Admin also defines which links on your site do not trigger Outbound Click events when using Enhanced measurement.
To learn more see Set up cross-domain measurement.