Best way I can explain it is that this is designed for modders primarily, as they will be most likely to use these values. This doesn't touch the base game.

However you can use this in the editor if you know how the breed editor works.

I didn’t understand anything, I was just looking for a mod that would allow infantry to live longer than now, withstand more damage, etc. to enjoy long battles...

They will by default have their normal values, this has no effect on the default units unless you tweak them yourself. This simply allows different health values to be used, as by default you can only use 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 750 and 1000 Health as options which were very weird and specific.

I usually play in the editor, that is, after installing the mod, my automatically active units will have more health, or do I still need some kind of setting?

Two methods you can use this, one is through the breed editor in the editor itself. The values are automatically added to the list of health options.

The second would be through creating your own units in set/breed/(faction and unit path) and applying these values.

Remember to add % to whichever value you set as the health, as it will not work otherwise.
As an example, 1% is 1 Health, 1000% is 1000 Health. Hopefully this helps answer your question.

how to use this mod?