Because "эта собака " is "this dog " or "that dog". "Это" is used to mean "that/this/it is" or as a demonstrative pronoun for neuter nouns. "Собака " is a feminine noun, so "это" will never serve as a demonstrative pronoun for it. Check out this guide if you need more help/clarifications :)
"sobaca" is Spanish for "female armpit" (armpits are always gendered male in Spanish).
wait i have a question...why are there many versions of "my"? this is not very related to this sentence but idk, i just want to know when to use the right version of "my"
The different versions for possessive pronouns in Russian were exhaustively discussed in previous lessons. Didn't you read the comments before? Anyway, for the first person, in a few words, they are:
- мой (masculine)
- моя (feminine)
- моё (neuter)
- мои (plural)