How to use, learn, and practice Dvorak layout with hardware external keyboards?
I solved this by installing Termux, which is a "terminal emulator".  This particular terminal emulator can install gtypist for you, which is a superb, well thought out free and open source typing tutor, well-known in the linux world.  gtypist is very easy to use, and you only need to know two Termux commands.  Just install Termux from the Play store, open a new session (if not automatically done) and type:

apt install gtypist


and press enter/go/return.  It should download and install, and might ask you to confirm.  After that, use the command:

gtypist


(and enter/go/return) to use the tutor.  Instructions for tutor will be on screen.  Happy typing!

If you have a dvorak keyboard, then great, this should work out of the box provided you android device language settings are correct.  Otherwise I found an app called External Keyboard Helper (Demo/Pro) to be extremely powerful in solving individual key map issues, as well as supporting every layout I've heard of.  I tried the demo on one of my devices and once in a while it inserts text "... DEMO VERSION ..." into what I'm doing, but pro version is only $1.99 as of this writing.

These apps also support other layouts, including Colemak, and gtypist is extensible, so you can install other lessons (not tested on Termux version, but I don't see a reason why it wouldn't work).