Samsung previously had a very bad rep for updates that went out to its unlocked smartphones sold in the US and while the situation has improved somewhat, it is still not good. Example: the unlocked US-bound Galaxy A51 is now starting to receive the update for Android 11 with One UI 3.1 on top.
That’s all fine until you remember that international devices got Android 11 back in February. It’s about a four month delay right there for you. In any case, the new Android version from 2020 is finally on its way to you now.
It arrives in the air as build version A515U1UEU5CUF1 and includes the security patch level from June 2021. It should find its way to all unlocked Galaxy A51 devices sold in the US over time, but in the meantime, if you are impatient, you can always go to Settings> Software Update and manually check if it is available to you. Who knows, maybe you’ll be lucky.