You have already seen that many expressions of feelings and experience use the Dative: ”Мне нра́вится...”, “Ма́ме хорошо́”, “Ему́ 5 лет”, “Мне ка́жется ...” etc.
The Dative introduces an indirect object of an action: usually the person whom the action is directed towards. An example would be a sentence like “I handed a package to my mom”: “my mom” here is a recipient.
Actually, this depends on the verb, just like in English. Some popular verbs of speech, writing or “giving” will use the bare Dative for the recipient: говори́ть, сказа́ть, писа́ть, чита́ть, дать, принести́ and so on.
По has an additional meaning, “apiece” or "each" : «Они́ взя́ли по три я́блока»=”They took 3 apples each”.
Plurals generally have only one pattern shared by all nouns. The ending only depends on the case, not the class of a noun:, «я говорю́ о дома́х, стра́нах, города́х, я́блоках, дочеря́х».
Only the Nominative and (especially) the Genitive have a number of different plural endings that depend on the class of a noun.