Where do Internal testers download Google Play Android apps?

The link of testing will be https://play.google.com/apps/testing/packagename and you can find that in Alpha/Beta publish tab.

https://play.google.com/apps/publish --> Select app --> Release Management --> App Release --> Alpha/Beta --> Manage --> Opt-in URL (URL for testing)

Updated to add tester (Close test)

Add testers

1. On the left menu, select Settings Settings > Manage testers > Create a list.
2. Type a name to identify your list of testers. You can use the same list for future tests on any of your apps.
3. Add email addresses and create list.
4. On the left menu,select Release management > App releases.
5. Next, to the closed track, select Manage.
6. Next to "Choose a testing method," select Closed Testing.
7. In the "Users" section, check the box next to the list you want to use.
8. List of will be notified and selected user can accept the invitation and test the app.

Here's how I found it (with help from Scar Coder's answer)

1. 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:

2. Expand the 'Manage testers area'

3. Copy the Opt-in URL: