Android Question B4XPages - how to proceed from B4J to B4A or B4i?

For the layouts. You'll need to open both B4J and B4A (or B4i) and open designer in both.
Open a new layout in B4a, copy the views from B4J to that layout and save it with the same name.
In the end you'll have identical layouts in each platform, although internally they are different.
(you will have to add the appropriate libraries in each platform - like XUIViews)

The next thing you have to do is add platform specific events. Look at the B4X basic language booklet to find out the differences.
C:\Users\willi\Desktop\Booklets\BasicLangage\B4XBasicLanguageV2_2.pdf

Android presents a special challenge with portrait/landscape issues. Search the Forum.
There will also be differences with respect to media, sound and video.

The real challenge always is to make your design look good with different screen size and orientations.
But you'll have to struggle with that yourself.

Usually that's all. But the IDE will alert you to other problems.

I always develop in B4J and switch to B4A at the end. My projects aren't large, but it usually takes about 2 hours.

I thought that chapter 8 Project with two pages explaining the different steps would have answered your questions.
This project begins the development with B4J, then the steps with B4A , change the package name, add the second B4XPage, copy the layouts etc and then the steps for B4i.