Emended hardware firmware is different.
Most people don't understand hardware.
Almost everything you use is not even source viewable.
Specially your raspberry pi.
Discussion
Even then, if the source was viewable but nobody looked, what difference would it make if it was closed source?
No-one is reviewing the whole Linux kernel + drivers + firmwares + microcode + etc. etc.
"Reflections on trusting trust - Ken Thompson" https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_ReflectionsonTrustingTrust.pdf