For the issues you are seeing with the layout of the app after downloading, see this post from Ioannis:
The default size of the screen on Thunkable when you are designing your app is 375x667px. If your device is not that same size, there may be some variation in how the UI renders. Because of this, I would consider using the Layout component:
This feature, which is similar to rows and columns in our Snap to Place builder, allows Creators to precisely position components within containers. The Layout component is a game-changer for quickly aligning multiple components, defining UI behavior across different screen sizes, and much more!