Preview vs final look

Hi, I haven’t compile an app for distribution yet, but I’d like to know what’s the best way to see how the final result would be. I’m asking this because I found some differences between what I see on Appgyver platform, the preview portal and the SAP AppGyver Preview app (Android). For instance, I have this row I’m trying to design.
1)Here’s how it look on the platform (the way I actually want to be):

  1. Here’s the preview portal (almost the same, just with a gray background on the right extreme):

  2. And here’s the Preview app (with all corners rounded, not just the exterior ones like I want):

I have similar issues with other components, such as the input fields (when I add borders to them I can’t see them on the app).

So, my questions are:

  1. What’s the best way to see what I’m working on?
  2. Is there a “best” screen size to work with? (I use the iPhone XR XS one)
  3. If I compile/distribute my app as a webapp, will it look like the one I see I’m working in? Will it have any restrictions (for example, in the Preview portal the mapview doesn’t work)?

Sorry for all the questions and thanks you all.

Looking at the actual apps is definitely recommended.

Get registered and setup on the app store / play store and you can test your app without any approval.

If Android, just load the apk directly, if IOS, use testflight.

Or, produce your webapp, and look at that.