Honestly, not too sure if I'm gonna sound really dumb here, but I got a used Samsung S9 and its stuck on Android 8 and I cannot figure out how to get it to even Android 9. Honestly any version above 8 would make it so much better considering that my S8 is running Android 9. Is updating it at this point even possible or is it too old to do that? Thanks.
You'd have to Root it - Yes it has a locked bootloader, but there's a reputable Dev on XDA who has figured out how to unlock the bootloader, but it's not free. I think that it's somewhere between $100 & $200 USD.
Then you should be able to flash a custom Rom. The S9 easily will work with Android 10
No. He doesn't need to root his phone as it should have received One UI 2.5 update since that's the newest version the S9 supports. Maybe an issue with his phone update but he could flash the newest firmware available for One UI 2.5 or Android 10.
Well obviously if there's an OTA available, that would be the best & easiest option.
Since I no longer have the S9+ I don't pay attention to what the latest firmware is from Samsung.
None the less, until fairly recently Root wasn't even possible because of the locked bootloader, but it's possible now to unlock the bootloader & flash custom Roms. I haven't checked, but I imagine that Android 11 is also available for the S9 series.
I've got an ancient Note 3 running Android 10 AOSP, so we know that the S9 series hardware is certainly capable of running Android 11.
