Engaged Session: This metric is the count of sessions that lasted longer than 10 seconds, or had a conversion event, or had two or more screen or page views.
Average Engagement Time per Session: This metric indicates how long a user has engaged with your page or app in a single session. It helps you understand how engaged users are and for how long they remain active on your site.
Engagement Rate: To calculate the engagement rate, GA4 divides the number of engaged sessions by the total number of sessions. Bounce rate is now the inverse of engagement rate (unengaged sessions/total sessions).
Segments and Audience Builder in Google Analytics 4
Google Analytics 4 makes it easy to create custom segments of users, sessions, and events.
With the Segment Builder, one can create a segment by defining parameters, conditions, selecting a date range and condition period. Segments must have at least one condition set to be saved. Conditions can evaluate dimensions, metrics, and events.
There are three types of segments:

user segments (subsets of users)
event segments (subsets of events)
session segments (subsets of sessions)
GA4 also includes an ‘At any point’ option that allows evaluation even if conditions are no longer met. Dynamic evaluation can also be enabled to capture changes in user data over time.