OK, so ignoring my earlier, light-hearted approach to this problem.
Legato is produced by slightly overlapping one note into another. You play, say, C, then press the D whilst still holding down the C. At
exactly the right moment you release the C. Release the C too early or too late and you do not have a perfect legato.
You continue up the scale in the same way.
Confused? You should be.
Playing the piano is an aural activity. Actually, this is true of any musical activity.
So, how do we play legato scales on the piano?
I provided the technical answer earlier. It is nonsense.
We play legato
anything on the piano by listening to what we are doing and adjusting our physical movements according to what we hear. If it
sounds legato then it is legato. If it doesn't, it isnt.
Use your ears, folks. They are what we play a musical instrument with.
Hope this helps.
Steve