Target apps through the Display Network
Apps are part of the Display Network, so you can show you ads in mobile apps by selecting "Display Network only - All features" when you're creating your campaign.
The ads in your Display Network and Search Network with Display Select campaigns are automatically eligible to be shown in mobile apps if you selected “All features” (the default option) when creating your campaign. Your ads may show in mobile apps when a mobile app placement matches the targeting that you've set for your campaign.
Promote your mobile app within other apps
You can also show ads that include an app install link within other apps — those are called app promotion ads. To learn more about showing ads for your mobile app in other apps, and which campaigns are best for your business goals, read about promoting your mobile app.
Monitor your ad placements
If the ads that are shown in apps are doing well, you should consider targeting apps through a mobile-only campaign — try reaching specific apps or app categories in the Display Network. To see which apps are showing your ads, go to "Placements" from the Display Network tab. Any apps that have shown the ads from the selected ad group will be listed there, along with any websites that have displayed your ads.