An engaged session (used to calculate engagement rate) is a session that spent more than 10 seconds on the website, had 2 or more pageviews, or had a conversion event occur. On the flip side, a non-engaged session (used to calculate the bounce rate in GA4), spent less than 10 seconds on the website, did not view any other page, and did not perform a conversion event.
In essence, bounce rate in GA4 is the inverse of engagement rate, and these metrics are both valuable in understanding if and how your users are engaging with your website.