Only those who know the URL of an app’s distribution page can access the page.
You can also set a password to limit access and provide additional security.

Additionally, in the case of the Enterprise/Flexible Plan, you can set accessibility to "Only team members who can access the application.

To set a password, please go to the app’s distribution page. On the right pane, select “Settings.” In the “Privacy setting” section, select “Require password.”

After selecting "Require password" in the "Privacy setting", "Password" field will be displayed. Input desired password and click "Update Settings" button to save and reflect the change.