Watch how the Sessions metric in Google Analytics 4 (GA4) differs from Sessions in Universal Analytics (UA).
Differences between Sessions in GA4 and UA
Sessions in GA4 and UA follow the same idea, with a few differences.
GA4 estimates sessions. GA4 estimates how many unique session IDs reached your website or app. UA counts sessions and doesn't estimate them.
GA4 ends sessions less often. UA ends a session after 30 minutes of inactivity, at midnight in the view time zone or when new campaign parameters start. GA4 ends a session only after 30 minutes of inactivity.
Counting sessions outside GA4
Each GA4 property includes the session_start event. When a session starts, Google counts that event and creates a session ID.
If you export GA4 data to BigQuery, count the distinct session IDs. That count can be further from the GA4 Sessions total than Sessions or Event count for session_start from the API.
See Troubleshoot session and engaged session count differences in GA4 for why Funnel totals can differ from GA4 and which session metrics to use.