So this is an interesting, and unfortunately tied specifically to your custom implementation scenario; so there's no real documentation....
The thing is, mobile app tracking has metrics like App Version and Lifecycle metrics which cannot be a part of your web JS tracking... but I understand that logically you would want to track those pages within your own "App traffic" definitions....
First, I assume you are passing the ECID values from the app to the webview pages, and that your website is reading those parameters and making sure to identify the user as the same ID.
Now, there are different ways to do this, but it would still be customized...
There are different ways to identify this.. you could use a custom parameter passed on Webviews to indicate that your App was loaded within the app (this of course might end up getting shared as a link which would impact your ability to track reliably).
Here is what I use, but it's a bit more involved. I actually used a trick used by a lot of social media apps, I had our developers append a custom value to the User Agent within the app. Basically, all webviews through an app use your default browser to render the content (the content will be identified as that browser / OS)... appending to the end a value like "[my app]" doesn't change how Adobe / Google / etc identify the app, but you can actually create a rule to check for "[my app]" and code in special tracking for anything loaded in this scenario. If on your website you see this User Agent variation, you can set a dimension that you use in segmentation to identify web from app (however you are determining that)
