What about old PCs? I inherently do not trust my phone.

https://en.wikipedia.org/wiki/Intel_Management_Engine

You can trust GNU/Linux phones though (Librem 5 and Pinephone).

You can read the article you're replying on. If the device uses the internet, chances are they're gonna be able to listen in.

That is way different from client-side [...] from applications though. I thought this was the case.