Backdoors in the CPU microcode, backdoors in the proprietary firmware of your motherboard / hard drives, backdoor through Intel Management Engine / AMD PSP. They’re all hardware level backdoors that can’t easily be disabled / replaced on newer systems.
There are only a select few of systems out their that can run a fully free BIOS with no IME, but those systems are about 15+ years old. In terms of freedom, we’re fucked. Even if you do switch to GNU/Linux, you’re still not entirely free.