When do users become exposed to a Firebase A/B test when no activation event is set?

I'm running an A/B Test targeting 100% of iOS users with specific versions using a regex to match versions 2.1.27 and up, here's the regex in case it's relevant:

2\.1\.([3-9].|2[7-9])

I did not set any activation event for the experiment; meaning I left that field blank.

Now my question is: Who becomes part of the experiment? Anyone who opens the app with a matching version? Anyone who starts a session with a matching version? Anyone who engages with a matching session?

So far, it says the total number of users that have been exposed to the experiment is 9,6K. I'm trying to set up a funnel that only includes these users but I can't figure out one that shows me anything close to that number in the date range of the experiment.

Firebase Support says:

Firebase will determine if a user will be part of the experiment when your app do the activatedFetch method.

That's the answer I was looking for.