How does GA4 handle ‘cross-domain tracking’ and what misconfiguration causes it to fail?

Question

Google Analytics GA4 — Hard

How does GA4 handle ‘cross-domain tracking’ and what misconfiguration causes it to fail?

Accepted Answer

Cross-domain tracking in GA4 requires configuring all domains in the 'Configure your domains' setting. GA4 automatically appends a _gl parameter to links between configured domains to pass the client_id. Failure occurs when domains are not added to the list, the _gl parameter is stripped by the destination server, or links use redirects that lose the parameter.