Without knowing what bootloader you are using, I have to speak very generally, but the stage1 bootloader in the MBR is very primitive. It is given, at the time it is installed, the absolute disk addresses of the next stage bootloader, which includes handling for at least one filesystem. Often that next stage bootloader is located in the unpartitioned space that immediately follows the MBR so that it does not move when filesystems are updated. That intermediate bootloader, working with the filesystem that it understands, loads the final bootloader, which implements all of the user-visible features and loads the actual kernel, initramfs, etc.
Note that the above is very general, and is probably not entirely correct for any actual bootloader.
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