Already rooted Samsung J5 - general inquiry

[AC Question]

Hello guys. I just bought a used Samsung Galaxy J5 (SM-J500FN) with android version 5.1.1. The previous owner told me that it is rooted. Nothing seems weird with the phone other than when i try to factory reset it it takes me to a "bios" kind window where i have to manually choose to delete all data (hard delete i think its called), that and when i normally turn on the phone, at the top left corner it says "set warranty bit kernel". I was just wondering how do i go about installing custom roms since its already rooted, and can i unblock the default carriers (T-mobile) block for other sim cards. When asked how he rooted it (what method was he using) he said he just used Odin. I dont know much about rooting so, whatever you can tell me as to the handling of the phone would be amazing. Any other guides to installing some custom roms and making the phone easier and faster ( i only want functionality, dont care much about the UI) would be greatly appreciated. Thanks in advance.

 

[B. Diddy]

Moved to the new J Series forum.