Where can I download the official ATSAM D21 bootloader and how can I burn it?
Google samd21 bootloader.
Answers all your questions.
In case of Arduino boot loaders , have a look here:
the official ATSAM D21 bootloader
Arduino boot loaders
I guess it depends on what atsam meant by "Official" ... ?
© 2020 Microchip Technology Inc.