What is the difference between ‘first_visit’ and ‘session_start’ events in GA4?

Question

Google Analytics GA4 — Hard

What is the difference between ‘first_visit’ and ‘session_start’ events in GA4?

Accepted Answer

The difference between 'first_visit' and 'session_start' events in GA4 is that 'first_visit' only fires once when a user visits for the first time, while 'session_start' fires at the beginning of every session, including the first. 'first_visit' is used to identify new users and track new user acquisition, while 'session_start' tracks engagement frequency.