Yes they are different. Listen. Мой = for masculine. Моя = for feminine. Моё = for neutrals. Мои = for plurals.
And all of them mean (my or mine)
***note: feminine nouns in Russian ends with я or а and usually ь. Neutral nouns ends with е or о. Other nouns usually are masculine.
Best wishes :)
Yes. The tips section for this lesson (not on mobile) describes this rule.
In short, feminine words ending in ь will have the ь be replaced by ы when pluralized and most masculine words that end in consonants will end in ы or и when pluralized. Less often, the masculine words that end in consonants will end in a or Я.
Additionally, singular nouns ending in a or Я become, respectively, ы and и. Singular nouns ending in o or e become, respectively, a and Я.
For these last two, I'd recommend memorizing the ladder/chain o -> a -> ы and e -> Я -> и.
Sadly, there are always exceptions but don't be discouraged. The goal should always just be progression from where you were at yesterday. Mistakes lessen over time.
If, however, your question is "When should I add ы vs и when pluralizing words that end in consonants?" I believe you may just have to consult a dictionary and memorize.
If anyone knows of any structure when choosing between adding ы and и for pluralizing singular nouns ending in consonants, someone please comment.
In russian does the possessive agree with the possessor or the possessed ?
For instance, in french you can only say "mon frère = my brother ; ma mère = my mother ( neither ma frère when a woman wants to mention her brother nor mon mère when a man is mentioning his mother is allowed) i.e. agreement with the possessed object and not with the possessor.
Some clarification please.