Let's review what these GA4 automatic events do

[sharminsumu]

Automatically collected events
Some Google Analytics 4 events are collected automatically, and you can't opt ​​out of them.

Here are some examples of events collected automatically:

session_start event
first visit event
user_engagement event

Start of Event Session
Two important things happen during each session_start event.

An infographic showing what happens during a GA4 start event.
Google Analytics generates a ig data session ID for the session: The ga_session_id event parameter now applies to all other events in the session. It allows GA4 to group these events for a session within the session.
Google Analytics generates a session number for the session: The ga_session_number event parameter counts the number of sessions that particular user has had on your site.
Pro tip

You can save the ga_session_number event parameter as a custom dimension to see this data when you look at your session_start event.

"Number of sessions" table in GA4
First Visit Event
The first visit event identifies new users to your site or app. If Google Analytics doesn't recognize a user by their browser cookie, the first_visit event fires.