The CIA's hacking tools were leaked in the Vault 7 leaks from WikiLeaks. They were fairly amateurish. The nearest component was some 0 days for smart tvs to turn them into wiretaps. It seems like the NSA really does get the focus for signals intelligence.

If those were amateur what does professional look like?

No, lol. They don't. Maybe Windows, because Microsoft doesn't seem to care much about privacy, but Apple doesn't mess around. And if there were backdoors in Linux, we'd definitely know about it, because people are actively looking for stuff like that.

This is just wrong. Apple's public position could differ from its actual one. And Hiding a backdoor (or hiding with the deniability of an innocent bug) in open source is possible. "we'd definitely know about it" is not how security works.

There have been exhaustive third party reviews of the Linux kernel, nothing is impossible, but a government backdoor would be extremely hard to hide from the level of scrutiny that codebase is put under.

I would certainly consider it unlikely enough that any suggestion to the contrary, without evidence, is just a conspiracy theory.

"we'd definitely know about it".

Yes, but extremely hard is not the same as impossible. How many security 'bugs' can we be sure were unintentional?

This is not just semantics. Complacency can come from over-confidence.

Could there be a backdoor on the hardware level?

I mean, yes, as we have already agreed, anything is possible, but I'm not in the business of believing things without evidence.