The state of embedded development in 2021

[Kerlin]

Yup that's right if you want your pcb to behave like a teensy for programming, then yes you need PJRC's pre loaded bootloader. I've put MK66, ie teensy 3.6 chips in the wild without any bootloader chip and just swd programming.

Thanks heaps for that, yes I do understand that a boot loader is not needed and the chip can be programmed other ways.
But I think that its not impossible to set it up so it is required, so was hoping to hear from some one who has done it.

In reality I would just buy the Teensy board as it is readily available if a replacement is required and I could not make it cheaper.
Just fit it to my custom board.
Was just looking to make 100% sure as I would not like to be stuck with it as a must have.