This xz backdoor thing reminds me of a story I heard from friends that worked at a tech company that made cell phones. They had a great coder that worked on the project, he had put in work as a contractor for a few months, and due to the quality of his work he was hired in full time. After two months he simply stopped showing up to the office.
An investigation turned up the following interesting items. His account had accessed all files including source code to *all* cellular projects - in that he had apparently downloaded a copy of everything. He had committed a large amount of contributions to the project he was assigned to. None of his paychecks were ever cashed. A wellness check to the house he had rented was performed and the house was completely empty. Per the landlord he'd paid for 6 months rent in advance in cash. Apparently he never physically moved in. No record for him nor his social security number seemed to check out. The guy was a ghost.
I was asked about recommendations on future prevention by friends who worked there - no idea how far they got in their investigation, if backdoors were ever found or even existed, or if the Feds were ever involved. The punch line? This was probably a couple of decades ago.
This shit is real, and it has been going on for a long time.