In GA4, a session is engaged when it meets at least one criterion: it exceeds the engagement timer (10 seconds by default), records a qualifying key event, or includes at least two page or screen views. The web-stream engagement timer is configurable, so check that setting before comparing reports.
Google excludes first_visit, first_open and session_start from the key-event route to engagement. Use the same stream settings and definitions when comparing periods.
Example
A visitor who views a service page and then a contact page qualifies through two page views, even without submitting a form. A single-page visit can qualify through the configured timer or a qualifying key event. Engagement is not the same as a lead or sale.