I'm guessing you were playing on a cracked server, meaning that you don't have to own the game in order to sign in. That's why you were able to sign in with any username, usually, it wouldn't be that easy.
I was playing minecraft today while using meteor client and I realized that you can use other accounts. I tried the admins account and what do you guess, it worked. I can use admin commands but not until I login to the account. Is there a way that I can figure out his password?
If you have some info on him, you could try to guess or if you're a bit savvy you could make a dictionnary and try to brute force. Anyway you're unlikely to find his password if it's strong enough
no. probably just got a cracked server like meme said but i would just buy one, they're cheap now