Does Apple's TestFlight application auto update apps that external beta testers have installed?

I can see from my iTunes connect control panel how many users are on each version of the app I have in beta. When does TestFlight update those apps? Do they have to open TestFlight and manually choose to update? Does it happen automatically when they launch the app they are testing?

I can't seem to find any answer on Apple's documentation.

Does Apple review TestFlight apps?

If you're sharing TestFlight beta builds with external testers, Apple requires a review of the first build of each new version. (Subsequent builds for a given version won't need a review, nor will any builds that will only be going to internal testers.)

How do I update TestFlight apps?

Open your email invitation or tap the public link on your device. When installing via email invitation, tap “View in TestFlight” or “Start testing” then tap “Install” or “Update” for the app you want to test. When installing via public link, tap “Install” or “Update”.

How long does TestFlight external review take?

Approval usually takes no more than 48 hours. Once Apple approves your version of the app, subsequent builds won't need a review until you change the version number.

They have to manually update it. A push notification and email is sent to let the user know that a new version is ready to be tested, but it's up to user to open test flight and install it.