Why can any song can be played starting from any note?

This is because most people have a sense of relative pitch, so as you say, as long as the intervals (tone distances) are kept the same, the piece of music is recognisable.

1. Can any song be played in any key?
2. Do songs always start on the root note?
3. Can a scale start on any note?
4. What does it mean when a song is played in a certain key?

Can any song be played in any key?

Yes you can transpose any major key song into any other major key. No, you cannot play every song on just white keys of a keyboard - White keys represent the intervals of the major scale in C (step, step, half, step, step, step, half).

Do songs always start on the root note?

A song can be in the key of X but not necessary start on the named note; however, it is likely to end on that note. If not, the ending will tend to be ambiguous. But when you ask about starting a scale on a different pitch, that's called a mode. For example, a C major scale played from D to D is called Dorian.

Can a scale start on any note?

You can play the scale beginning on any note, and if the song is in the key of C (in other words, the harmony is in the key of C), then the scale will work just fine.

What does it mean when a song is played in a certain key?

Most music is in a particular key. If we say that a song is "in the key of C," this means that the pitch C sounds like the most stable “home note” (or tonic) for the song. Likewise, most songs use notes within a particular scale — a collection of notes in order from low to high.