This is a repository of BIGTREETECH bootloaders. The originals are stored under the bootloader_bin/backed_up_original_bootloaders/ directory. This repository also has a bootloader for the SKR E3 TURBO to fix the high pitch sound coming out of the CR10_STOCKDISPLAY it is located on the bootloder_bin/compiled_bootloader_bin/ directory.
I can't see pdf files with a description of how to flash, github swears at the pdf - says they are damaged