@nijupp24 I've attached a binary for a minimal SAMD21 bootloader for a device with 16KB RAM with only native USB support. It has neither LED status outputs nor auxiliary serial port upload. It also boots crystalless, irrespective of whether your board's got a crystal or not.
I've successfully tested it on my SAMD21G18A based custom board, using it to upload a "Blink" sketch, including double tap reset to put the microcontroller into bootloader mode. So I can confirm that it all works.
samd21_sam_ba.zip (4.7 KB)