Comment by anderskaseorg

The mechanism is a clever application of quines (self-reproducing programs), first explained in the classic lecture “Reflections on Trusting Trust” by Ken Thompson:

https://dl.acm.org/doi/pdf/10.1145/358198.358210

Russ Cox obtained the actual code for Thompson’s compiler backdoor and presented it here:

https://research.swtch.com/nih