Beginner German Question

I'm relatively new to learning German and noticing different ways to say the same word for example,

"meine mutter" "mein vater" which I understand is due to the pronouns I think?

But anyways, why is it "die katze" and not "der katze" and why is it "der elefant" and not "die elefant"

Would really appreciate any advice/tips to help me understand :)