MacOS - how to save custom keyboard layout?

Question

macOS

Given - Macbook, American.
Worth - Catalina. Layout - "Russian PC". In it, in this version, "Ë" finally moved to its rightful place, to the left of the one, to the tilde "~".
It is necessary - to transfer this layout (from the correct Ë) to High Sierra (a saving downgrade is needed).
Question - how can this be done? Export layout from Catalina and install in High Sierra?

Answer 1

why? there is a drop-dead ukelele software that allows you to create whatever layout you like.
I personally use phonetic with modifications.
which successfully migrates with me since 10.7.