I dunno. My computer has at least 1 hardware backdoor that I know off, but that I just can't get hardware without any equivalent exploit.

My OS is developed with a set of tools that is known to make code revision about as hard as possible. Provides the bare minimum application insulation. And is 2 orders of magnitude larger than any single person can read on their lifetime. It's also the usable OS out there with best security guarantees, everything else is much worse or useless.

A browser is almost a new complete layer above the OS. And it's 10 times larger. Also written in a way that famously makes revisions impossible.

And then there are the applications, that is what everybody is focusing today. Keeping them secure is close to useless if one don't fix all of the above.