To boot legacy items, the BIOS copies to ram the MBR (Master Boot Record)
which is the 512 bytes at the beginning of the drive, which is outside of any file system. and executes it (the MBR copy that is in ram)
Legacy boot does not understand files or file systems.
UEHI does understand files and file systems.
UEHI finds the boot configuration files, usually in a folder named boot
The UEHI boot files are normal files that can be copied and moved and deleted and edited.