You can replace the Operating System and even firmware on a lot of devices. This is cool, as some companies, such as Apple, deliberately slow down the functioning of your device when they feel it is time for you to 'upgrade' it to a newer one.
In PostmarketOS, mainline means you can use the standard Linux kernel, instead of a fork. USB Networking means you can connect the device by usb cable to your PC and telnet into it, etc. Just hover over the "Features" for example here: