Ask HN: How do manufacturers ensure the absence of hardware backdoors?

I came across this old thread which discusses a ban of lenovo computers by multiple intelligence agencies, citing hardware-level backdoors: https://www.reddit.com/r/netsec/comments/1j78jk/lenovo_pcs_banned_for_use_by_nsa_due_to_chinese/

This made me curious--how do benevolent hardware manufacturers ensure that their products aren't modified by third parties before/during production?