https://www.udemy.com/course/google-ads-masterclass/learn/lecture/17340954#overview
App ads focus specifically on having your app downloaded from the app.
App extension is an extension of the primary ad with the ability to download your
application.
To add an app extension, click on “More asset types” and select “+Apps”.
Choose if you're going to be on Android or iOS wherever you have your app.
And if you do have your app on both Android and iOS you simply set up two extensions.
Next, search for your app and click to add it.
Then, edit the “Link text” accordingly.
⚠️You're not going to be downloading any apps on your desktop.⚠️
Other available options:
App URL options — lets you add tracking options
Advanced options — allows you to specify a schedule for this particular extension
Once you’re done, click on “Save”.