OpenRA

Is there any modding guide for current version? And how is it even working? I couldn't even change rules, I made tweaks and stuff even like cost decrease but it wasn't really in the game. Can somebody explain how exacly openra reads stuff?

The game reads stuff in the defined order from mod.yaml. Beyond that, you only need https://github.com/OpenRA/OpenRA/wiki/Traits-(playtest) or https://github.com/OpenRA/OpenRA/wiki/Traits (depending on version). Everything onwards is usually a no-brainer.

If you created a new yaml and thrown your stuff there, then you have to list this new yaml in the mod.yaml. If you edited a base mod yaml then the changes should pop up during playing.

Then it seems that I have a problem, without changing anything else or making new folder, from old c&c mod I changed cost value of mammoth tank and it haven't changed (of course I've saved). What can make such an error?
Edit: works for playtest but not for normal version

The hand-written https://github.com/OpenRA/OpenRA/wiki/Modding-Guide is sadly way outdated.