This method works because boot.emmc.win is often just the raw block device. On many devices, the boot partition already a valid boot image starting at offset 0. But some OEMs add extra data at the end (e.g., Samsung signature footer). Manual dd can trim extra bytes.
In many cases, a "conversion" is simply a matter of correcting the file extension to match the expectations of your target software, as both files are often identical raw byte-for-byte copies of the boot partition. Распаковка boot.img и system.img - 4PDA boot.emmc.win to boot.img
A: Sometimes, yes—if your device has no extra footer and TWRP dumped a pure boot image. Test with file command or hexdump first. Manual dd can trim extra bytes