What exit rate means
The metric always looks at one specific page, never at the whole site. Every session ends somewhere, so every session produces exactly one exit. What matters is which pages the exits pile up on and whether that ending was the intended one.
The usual confusion is with bounce. A bounce describes a session that starts and ends on the same page with no further activity. An exit describes the end of the session no matter how many pages came before. Every bounced session also produces an exit, but most exits are not bounces: they follow a journey that may have been long and worthwhile.
Google Analytics 4 adds a wrinkle, because bounce was redefined. It no longer means «only one page viewed» but the opposite of engagement rate: a session counts as a bounce when it clears none of the engaged-session thresholds, namely lasting longer than ten seconds, firing a key event, or collecting two or more views. The exit, by contrast, stayed a fact about sequence rather than a judgement about the quality of the visit.
That is why the two metrics answer different questions. Bounce asks whether the visit ever got going. Exit asks where the journey came to a stop.