Sounds like you’re left with almost no modern conveniences or technology.

You are right, I forgot one exception. I do use a Google Phone because these have the best support for alternative operating systems. I replaced the Stock Android which is filled with Google spyware with GrapheneOS, an entirely open-source version of Android which makes many privacy and security improvements and gets rid of any Google services. Watch this video to learn more about it: https://piped.smnz.de/watch?v=yTeAFoQnQPo

On my laptops/computers, I exclusively use Linux. I refuse to touch any bloated, ad- and spyware-infested Windows computer. Yes, even at work. As a DevSecOps and Senior Linux engineer, I am free to choose whatever operating system I want. Free and open-source software is the way to go.

I use Firefox (LibreWolf, a better version of Firefox to be percise) with DuckDuckGo as my search engine, I watch YouTube videos through Piped, I use Signal as my messenger, I use Proton for Email, I self-host Nextcloud for my files, contacts, calenders, etc.

For entertainment, I simply pirate the content which gives me much more freedom compared to shitty streaming services like Netflix. That way, I also don't have to give these shitty corporations any money. I use the open-source Jellyfin media server to consume content from my media library. I do the same thing using Navidrome for my music.

Regarding Social Media, Lemmy is probably the only thing I use. I don't need Facebook, Instagram, Twitter or TikTok.

And the list goes on and on. I could write an entire book about avoiding proprietary software and shitty corporations.

ensure you are truly not using any of these companies or their subsidiaries

I do everything in my control to avoid it. I can't 100% guarantee it, but that's also not my goal. Not giving them my money or my data is enough for me.

Or even if a mom and pop shop down the street is using technologies, software, or appliances from one of these companies to run their local business…

Well, that's out of my control.