I assume, you're wondering about ''это'' (why not эта in the feminine form or этот in the masculine). In those sentences, это is a conjunction (NOT a demonstrative pronoun этот, эта, это), which connects two nouns in the compound nominal predicate. Eg.: Елизавета II это королева Великобритании; Принц Филипп это её муж. As a conjunction это doesn't have singular/plural form or gender: Елизавета I и Елизавета II это две английские королевы. You can replace это with a dash: Елизавета II -- королева Великобритании.

We usually use "это" when we give a definition to a word or a concept so the sentences would sound a little funny if it wasn't a video made for little children. For someone who already knows "мама" and "папа", but not "бык" or "корова" these simple sentences could be a way to explain that "бык" is a male animal and "корова" is female. Simple analogies:) If you were talking about people being a mother or a father you wouldn't normally use "это". You would say something like "Мария - мама"/ "Александр - молодой папа".

almost all living creatures have male and female gender . =) cлон(он), слониха (она), слоненок (ребенок) пёс (он) собака(она) щенок (ребенок) - Sometimes we can use the feminine gender to refer to a family of animals (like this) кот (он) кошка (она) котенок (ребенок) Тигр (он) тигрица (она) тигрёнок (ребенок) Ворон (он) ворона (она) воронёнок (ребенок)

Everything is correct. Бык = bull, Корова = cow. Bull is father, cow is mother. What's wondering you?