"obrigado" is for the Men! If you are a man, you say thank you "obrigado" and If youre a woman you say "obrigada"

I do not know.

I think there could be two reasons.

My first assumption is that endings of the word can be different depending on whether the person speaking is a man or a woman. Like it can be seen in Ukrainian and Russian.

My second assumption is that the endings depend on whether the person that we talk to is a man or a woman. Which can be seen in Hebrew.

Great, you had good ideas. The first is the right one. I am a man, thus I say "obrigado", you are a woman so you say "obrigada". Because this words came from a phrase "Sinto-me obrigado (a) em retribuir...". That's why we have this difference. Congrats.