>My mom always asks me if she’s supposed to double or single click. But she can use a smartphone UI without my help.

That's honestly just an old person thing to do, usually coupled with a huge helping of learned helplessness "I just don't get these newfangled computers" and refusing to learn anything new. Show them right click one time and they'll forever ask if you if something that they've always double-clicked on requires a right click.

If that were the cause it’d apply universally. But the point is that she can use her phone to a much higher degree of independence, despite fat finger issues, much smaller screen real estate, and lack of hover effects, double clicks and right clicks. I always find new apps and figuring things out, but struggling to explore and learn on the computer.

To add on top of that, the double click navigation isn’t even consistent on the same platform. In the sidebar on desktop you have single click navigation even on the same type of entity (directories). Look at the younger gens who only used phones. They struggle too.