There’s no reason for kids under 6 to ever look at a screen.
My own kids haven't done anything that impressive, but they've certainly enjoyed watching a lot of videos and it doesn't seem to have done them any harm. Doesn't get in the way of doing non-screen things either.
I’m not sure I can exactly explain why, but touchscreens are fundamentally different from a developmental standpoint. Also low res screens don’t seem to have the same effect. High resolution Touchscreens are aggressively addictive and encourage a certain kind of ultrafast loop that is simply not a dynamic on other devices.