What should I put in an App?
Whatever makes up the same product: its website, API, workers, mobile app, or anything else you ship together. Different stacks are fine. If your customers think of it as one thing, it probably belongs in one App.
Every app has its place
Your product is rarely just one thing. Group its frontend, API, workers, and mobile projects under a name that makes sense to you, then see how the whole thing is doing without mixing up the parts.
Do the projects get mixed together?
No. Each project keeps its own errors, releases, settings, and DSN. The App simply gives you one place to see their recent activity and spot which part needs attention.
What if I change my mind?
Move a project, leave it on its own, or delete the App altogether. The grouping goes away, but your projects and error data stay put.