this is pretty old by now but still very relevant. people dont look at this enough but with rising chip complexities for TPU units etc. and a shift towards poorly documented hardware like NVIDIA gives this problem new fuel.

Domas (and maybe his team or colleagues?) has put out shit tons of very interesting materials over the past years on advanced malware, implants and things like Cantor Dust which are amazing things to dive into.

using his own cpu fuzzer, msr fuzzing techniques etc. he has found, reversed and implemented attacks through hardware bugs and backdoors.

It cant be confirmed if a backdoor is malicious or for debugging but essentially the capabilities gained through them are what is important.

These techniques he shows throughout his videos are not super tricky to replicate and I can recommend people who have interest to dive into it, reproduce things and try to help in this domain to raise awareness and findings.

Another good avenu is: Defcon 21 - Decapping Chips The Strike Easy Hard Way

People speak about supply chain issues in NPM and Pip etc. but these are much more severe and hard to detect.

Almost no one looks at it. Most vendors totally ignore it because you cannot sell products against it. (if ud detect it u need to trash the hw so its not handy... for sales...)