Here's how I found it (with help from Scar Coder's answer)
Log into the Google Play Console and select the app from the list, then click the App releases option in the left sidebar, then click the 'Manage' button for your internal test release:
App should come to the published status before the Opt-in URL is enabled. You can get your app status in the small '?' symbol or in the drop down box location above the page. Google takes some time to review the app before publish after you submitted it.
https://play.google.com/apps/publish --> Select app --> Testing(in Release Part) --> Internal testing --> Testers and scroll Down and in How testers join your part take link of your app by clicking copy link part
As per the new Google Play console, you can share your app to internal testers by visiting internal app sharing upload page, Upload a new appBundle and share the link.
You can manage your internal testers under internal testing in the play console.
With internal app sharing, you can quickly share an app bundle or APK with your internal team and testers by uploading an app bundle or APK on the internal app sharing upload page and generating a link. When sharing your app this way, you can restrict access to email lists only or allow anyone whom you share the link with to download.
did not install the old version but the most recent version from the store. Even though the account was registered as "Tester" in "Internal Testing" in the Google Play Console and the developer feature "Internal app sharing" was turned on in the Playstore settings on the tester smartphone.
My solution was to clear the cache of the Google Playstore app.
Then, I had to become a developer in the Playstore again (by tapping "Settings / About / Play Store Version" several times) in order to activate "Settings/General/Internal app sharing" again.
Afterwards, clicking the link worked fine. And I got the longed-for message "This version of this app () may be old. Turn off auto-update for this app to keep it from getting updated." and could perform my tests.