when we talk about հոլովում (declension), we categorize them by how the noun inflection changes in the genitive/dative case (because only the gen/dat changes between types of declension). so ի հոլովում, means that we add an ի in the genitive and dative case; the rest of the cases don't change.
at least, that's the case for modern armenian. classical armenian, of course, absolutely had to make an already complicated language even worse. aside from declining adjectives too, classical armenian had a few extra things going with its nouns.
they still had ի, ոյ, ոջ, օ, եան, ուան, but also այ, ին/ան (which evolved into -ան), and some irregular stuff that modern armenian inherited.
of course, that's not to say that everything is irregular. again, we categorize these declensions by their change in the genitive and dative case.
for example աթոռ, belonging to the ոյ հոլովում, becomes աթոռոյ
որդն in the ան հոլովում becomes որդան
some nouns are one type in the singular and another in the plural; կամ -> կամի; կամք -> կամաց
the plurals have their own funky stuff and there's also a unique set of rules for nouns that end in ր and ղ. krapar.org does a way better job of explaining it so i think it's worth it to check out their page on the topic
