Until your Custom Audience from mobile apps has collected 100 X users, it'll show as "Audience too small" in your Audiences tab. Your Audience will change to "Ready" once it's large enough, after which you can use it for targeting in campaigns.
How do I build an App Activity Combination Audience?
App Activity Combination audiences allow you to combine multiple App Activity Audiences, and set time conditions on them. They allow you to target hyper-relevant user buckets, helping you drive your app's final action. Please note you can not create App Activity Combination audiences out of non-mobile app-based Custom Audiences.
Go to Tools > Audiences > App Activity Combination
Fill out the conditions. You can choose from your existing mobile app events, and combine or exclude them.
Create your App Activity Combination audience!