Technically, that's correct (which means it's the best kind of correct).
But... I'd add that doing that well isn't easy. I frequently find cases where the mobile version simply removes features the desktop version has. And tablet version are very very rare. They tend to be either just the desktop or mobile version. Rarely is someone designing for tablet.
And that makes sense. Designing 3 different sites to all be the same feature-wise isn't trivial. Then you throw mobile apps on top of it, and suddenly it becomes much harder.
Can it be done? Sure. But I wouldn't say it's trivial to do it well.
But... I'd add that doing that well isn't easy. I frequently find cases where the mobile version simply removes features the desktop version has. And tablet version are very very rare. They tend to be either just the desktop or mobile version. Rarely is someone designing for tablet.
And that makes sense. Designing 3 different sites to all be the same feature-wise isn't trivial. Then you throw mobile apps on top of it, and suddenly it becomes much harder.
Can it be done? Sure. But I wouldn't say it's trivial to do it well.