How does GA4 define a ‘new user’ and what are the potential accuracy issues?

Question

Google Analytics GA4 — Hard

How does GA4 define a ‘new user’ and what are the potential accuracy issues?

Accepted Answer

GA4 defines a new user as a user whose first event is in the period, identified by client_id (cookie) or user_id. Accuracy issues include cookie deletion resetting user identity, private browsing creating new client_ids, iOS 14+ restrictions, and Google Signals thresholding.