Confused about xiao 52840 ble sense versions 1.0 vs 2.6.1 - what is the bootloader?

I have a couple of XIAO BLE SENSE on the way, but was reading up on the two versions and confused. There is version 1.0 which works with (comes with?) the Adafruit bootloader and version 2.6.1 which works (comes with?) with the NANO 33 BLE Sense mbed bootloader. Which bootloader is loaded by default?

And can you swap out bootloaders fairly easily: for example want to have Adafruit’s n52840 bootloader vs mbed’s BLE 33 Sense bootloader.

This is our factory firmware.

Thank you very much.