2.15The difference between a key and a chord

This is worth making completely clear.

C major chord

means:

C E G

Three notes.

But:

key of C major

means the tonal system centered around C, primarily using:

C D E F G A B

and its native chords:

C Dm Em F G Am B°

So if somebody says:

“The song is in C”

they do not mean the song consists of one C chord.

They mean C acts as the harmonic home.