Internal tester

• An user with AppleID added to the Apple Developer Account
• He can be invited via email as an internal tester of any of the apps under that account
• He sees all of the builds immediatelly after upload to the App Store Connect

External tester

• An user with valid AppleID
• He can be invited via email or via public link as an external tester to any of the apps under that account
• He sees only the builds intented for external testers after they pass beta review by Apple
• Beta review is usually faster than normal review and currently is only triggered by version number bump (not build number bump)

TestFlight App

• TestFlight in AppStore
• It doesn't matter which AppleID you are using with your iPhone, you can still accept invitations from other AppleIDs
• Just accept the invitation from the email of AppleID that was added as an internal/external tester