Weird Start-up asking about OS

Today I booted up my phone after losing power, and it had instead of a lock screen a blue screen warning me about installing a custom OS. I have never even looked at getting a custom OS, obviously I denied the installation but this has left me wondering if this is a bug or if I have a virus or some other malicious activity on the device. The phone is pretty beat up, so I am considering just replacing it and moving all my important stuff onto external storage- if it is a bug is it readily fixable? Will it carry with any files (or even the sim card? idk these things lol)? If it is malware is there any chance of saving the phone or is it probably best to just get a new one? I dont want to risk bank details or pictures being sent off ://, side note - nearby share turns itself on when I turn on the phone aswell, but when I turn it off a popup appears asking about turning it on so I dont know if it is really off or on anymore idk if this is in anyway connected.