Below images show the notes used while playing Alaipayuthe on the guitar, from the C root.
Alaipayuthe is based on Raga Kanada, which is derived from (janya ragam of) Kharaharapriya, the Dorian mode.
Alaipayuthe played with C as the root will have the same notes as C dorian mode / scale. Try playing the B flat major scale and you will realize that it has the same notes as we use to play Alaipayuthe in C root. This is because the C dorian is the second mode of B flat major scale.
Once you realize that only select frets on the guitar are used to play the entire song, it gets easier. You won't doubt if the video shows playing on the first string 7th fret or a 6th fret. You will know that it should be the 6th fret since 7th is not used, 8th is the next fret.