Shell chords are primarily built using the 1, 3, and 7 notes of a chord. The video explains that while the 5th note is important for defining a chord, the 7th note provides crucial information about its quality and "home base," making the 1, 3, and 7 the essential components for a shell chord.
This video focuses on the concept of "shell chords" as a highly efficient way to learn and play guitar. The speaker emphasizes that mastering these chords can unlock the ability to play almost any song and significantly improve musical understanding, making it a valuable investment of time for guitarists of all levels.