RIgidbody sphere falls through plane

For me adding "sphere collider" component fixed exactly this problem during following the tutorial.

Not sure why rigid body was not enough.

I had exactly the same problem, and it turns out that I didn't check the plane collider. My problem was solved only by fixing the plane collider, but if "the ball may fall off from the plane" does not matter so much, checking sphere -> rigidbody -> constraints -> freeze position Y will lock the ball in the same height while unchecking sphere -> rigidbody -> use gravity removes the whole "falling down" thing.