Hi thanks to @jojodu34 I have started to understand the secrets of ABF.
I followed this tutorial and wanted to be able to build my own package. I guess then if I’m able to do it, we should be able to get enough informations to make a great official documentation soon 
So I made own repo here for building a OM Lx version of YACreader, a Qt 5 comic/images_archive reader.
After help from @tpg I could adapt a spec for Fedora made under OBS ot OM Lx (I’ll explain later the process, the fedora spec was here)
I figured out that I also needed a library called unarr to manage decompression of files. I could have built it statically, ie include the library inside the source of YACreader, but found more challenging and instructive to respect OMA policy to separate libraries.
So I alsa had to adapt the Fedora libunarr spec (found here), the final result for OM Lx is here with the massive help of @fedya. The result is here,
On their side, both @Colin and @bero helped me understand how to manage compilation locally (and to find the correct dependencies).
These packages compiled correctly on my computer, but I’ll soon need to understand the process inside ABF (including testing, kahinah etc)
So I’ll start two threads, one about making build tests locally, and improve the build requires, and the other about improving specs, especially when it comes to a library where should be provided several packages (lib, lib64 and devel packages)