Smartphones should really be used for looking up information quickly at a glance. When you need to do something for longer periods of time, you should use different devices. To prevent nearsightedness/myopia, you can use AR glasses, VR headsets, projectors, TVs, etc. instead of using monitors or reading physical books. The aforementioned devices keep your eyes relaxed (accommodation-wise) over long periods of time.
It could be both. And focus distance may matter. Like, books may also cause myopia. Reading books outdoors may be a bit better than reading books indoors. Using screens at a distance could also be benign.
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