SAMD21 Bootloader

Hi,

I am looking at the Arduino Zero and I want to build a PCB that uses the ATSAMD21G TQFP package, but I am having trouble finding out how to burn a bootloader once I have the chip itself. Is there any reference available?

Thank you

You'd use a JTAG/SWD debugger like an Atmel-ICE, with host-side software like Atmel Studio or OpenOCD.
See the datasheet, I guess. And/Or maybe the ARM SWD/JTAG/DEBUG specification(s). JTAG and SWD are standardized, but actually writing flash are per-vendor details.