The bootloader should be in the following holder
“C:\Users\XXXXXXX\AppData\Local\Arduino15\packages\Seeeduino\hardware\samd\1.8.4\bootloaders\XIAOM0”
If you have more than one SAMD21, it is very easy to use DAPLink from the link below. It also includes the bootloader files.
“How to unbrick a dead XIAO using a XIAO(DAPLink) and OpenOCD”
Also, if you use ST-Link, post #39 at the following link has everything you need.
“How to unbrick a dead XIAO using ST-LINK and OpenOCD - #40 by msfujino”