All memory is accessible. Buster is the new version of Rasbian. There is not a plan to update Rasbian in the near term to 64 bit per Simon Long (senior principal software engineer at RPi) who says at this link it will be changed when:
someone comes up with a convincing improvement resulting from a move to 64-bit which justifies the loss of backward compatibility which it entails. Which no-one has yet managed to do.
There are other flavors of Linux which may well implement 64 bit. See discussion at link above.
