And I wanted to change Iranna's starting skills, but in the end I realized my complete incompetence - I couldn't even figure out where to look for the files where this could be edited. =\

well, if you're looking for Iranna, you need to find a mod that changes Iranna, and then look at which files were changed in it - that's how I used to search

Iranna's file: MapObjects\Dungeon\Urunir.(AdvMapHeroShared)
skills are written like this
<skills>
            <Item>
                <Mastery>MASTERY_BASIC</Mastery>
                <SkillID>HERO_SKILL_LEARNING</SkillID>
            </Item>
        </skills>
        <perkIDs>
            <Item>HERO_SKILL_INTELLIGENCE</Item>
        </perkIDs>

I usually look up skill IDs through the map editor - I place the hero on the map, give him the needed abilities (to enter the hero and give him skills, you need to select him and press space), and then I look at the IDs as shown in the screenshot (this window opens automatically when the editor is launched)