[android] Where do alpha testers download Google Play Android apps?

Google play store provides closed testing track to test your application with a limited set of testers pre-defined in the tester's list known as Alpha Testing. Here are some important things to be considered to use alpha testing.

Important

After publishing an alpha/beta app for the first time, it may take a few hours for your test link to be available to testers. If you publish additional changes, they may take several hours to be available for testers

Managing Testers for Alpha Testing

1. The Screenshot is most recent as of answering this question. You can see the manage testers for closed alpha testing, You can add and remove tester one by one or you can use CSV file to bulk add and remove. The list of defined email addresses will be eligible for testing the app, here you can a control whom to provide the app for testing. Hence, this is known as Closed Testing.
2. You can see the link(washed out by red line), once your app available to test, your testers can download and test the app by going to the below-given link. For that Google will ask once to the tester for joining the testing program. Once they have joined the program, they will receive an app update. As stated by store, it may take 24 hours to make an app available for testing.
3. Once your app available, Your invited testers can join the test by going the link https://play.google.com/apps/testing/YOUR PACKAGE NAME

Managing App Releases

1. After the Manage testers card, there is a card for manage release, from here you can manage your alpha releases and roll-out them to production by clicking the button at the top of the card once they well tested. This process of rolling out from testing to production/public is known as stagged roll-out. In stagged roll-out, the publisher publishes by the percentage of users, to better analyze the user response.
2. You can also manage multiple alpha release app versions from here, at the bottom of the screenshot you can see that I have once more apk build version being served as alpha test app.

Managing Closed Track Testing Availability

1. Apart from the user based control, you have one more control over the availability of the app for a test in the country. You can add limited countries tester to the app. suppose your list of the testers are from multiple countries and you want the application to be tested in your country only, rather removing testers from the testing list, you can go through Alpha Country Availability. It gives more precise control over testers.
2. Here, In Screenshot, my app is available worldwide states that my testers (from testers list) can test the app in all countries.