The only platform that you can still get the hood open now is Linux. I personally prefer Arch Linux or OpenWRT depending on the hardware and expected use for a project.
But even with Linux you need to choose carefully as vendors work to close even the many products built upon Linux. Just buying hardware with Linux doesn't mean it's open enough to be useful for example: Android as generally sold. AOSP is the exception.
If you want to intro someone to "old skool" look at the Raspberry Pi platform or OpenWRT. NOTE: even those have proprietary components and BLOBs of proprietary binary only code in almost ALL circumstances.
General consumer computers like Windows PCs and Macs are becoming like current cars with less and less access with each new generation. You can work on them yourself less and less. This is driven by both manufacturers wanting more control particularly as advances in CPU power have slowed and government regulations that lock down what were previously tweak-able components like WiFi router hardware.
Right now the most interesting stuff is in "do it yourself" IoT devices. But you better "do it yourself" rather than simply buy it. Otherwise it is likely "ownable" by someone other than you straight out of the box or ALREADY owned by the manufacturer who is spying on you using it. And given that there are no monetary or legal penalties for the complete lack of security this is unlikely to change regardless of various government solicited reports.