Both USB and bluetooth are digital signals capable of error recovery so apparent interference is reduced.
In addition to this usb uses twisted pair and shielding and uses digital signals which makes it far less susceptible to picking up interference.
While bluetooth uses frequency hopping spread spectrum which limits the amount of time interference matters and guard bands which limits how much interference can degrade a signal.
An aux line in uses a fairly large copper conductor to carry a signal but this signal is not shielded so its susceptible to interference, hum and popping. This is further complicated by the fact that USB powered line-in is usually cheap and may not have a shielded amplifier within either.
Yes USB speakers are a thing. Logitech S150 are an example. The marketing lingo appears to be "digital" if you want to look for them.
1Not answering, but this depends on tons of things from my experience. The reality is that even a “pure” digital setup will have to go analog at some point. So knowing that depending on how bad the interference is, some setups won’t help. I live in the U.S. and have a neighbor with a line-boosted CB setup. You know what that means? Even if I put headphones on my iPhone I can hear his yammering. Through the phone speaker, it doesn’t seem to exist but that small bit of wire from the phone to my head is enough to pick it up. – JakeGould – 2017-11-22T02:14:20.343