Signal itself is a backdoor I guess since it pretends to be "end to end encrypted" instead of warning you about the backdoors in your device, but I was talking about the backdoors in the devices themselves, at the hardware level.

Takshashila institute in India has a paper about how modern electronics all have backdoors, if you only trust academic sources, but you shouldn't only trust academic sources - before they released that paper, it was easy enough to just observe the lack of transparency around consumer device manufacturing, and the lack of reason for it other than to have backdoors.