Linux

I think the biggest fundamental concept for any computer regardless of operating system is filesystem hierarchy. The concept of nested folders is core to using a personal computer, but for the last two decades UI/X teams have done everything in their power to obscure and abstract it away. Many younger people conceptualize the storage on their device as just an amorphous blob that apps manage autonomously. Windows is starting to go this way as well with OneDrive being sold as the way to manage all your data, but on Linux the file system is still king.

Your mom is presumably old enough to have some experience with desktop PCs, so hopefully that basic hurdle is already cleared. And honestly once someone is at that level of base competence, along with basic interface concepts like how to use a mouse and keyboard, clicking on icons, use of a web browser etc, with the right distro you really don't need to explain much else. There might be a few quirks of the UI to explain depending on what you choose, but most of that can be handled by just watching them use the computer for a bit, and/or asking them to give you a list of questions and annoyances after they use it for a few days.

The biggest difference is one that most "I just want it to work" users will actually love, and that's relearning how to install software. Having one central location to install verified software from is a change from the wild west of downloading installers from the internet, but it shouldn't be a difficult transition. Most people these days don't even install software beyond maybe Zoom, so you can probably get away with just installing any third party software they need in the initial setup.

I recommend an immutable distro like Fedora Silverblue, at least if a) you're setting it up and are reasonably technical, and b) you don't want to go over and help them fix stuff often. I set my mom's laptop up with it 4+ years ago and she's only had one problem since then.