Harmony is two or more notes played together at the same time. As soon as there is more than one pitch sounding at a time, you have harmony. Even if nobody is actually playing chords, or even if the notes are part of independent contrapuntal lines, you can hear the relationship of any notes that happen at the same time, and it is this relationship that makes the harmony. The study of harmony involves chords and their construction and chord progressions and the principles of connection that govern them. Below are some videos providing instruction in harmony.
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Leading Tone diminished 7th chords: 2nd inversion
Leading Tone diminished 7th chords: 1st inversion
Triads in Inversion in Music
Figured Bass in Music: Chorale Style
Voice Exchange in Music Harmony
Cadential 6/4 Chords in Music
Deceptive Cadences in Music
Subdominant Chord Options: ii6/5
vii6 Chords: Passing Chords in Music
Dominant 7th Chords in 1st Inversion: V6/5
Dominant 7th Chords in 3rd Inversion: V4/2
Dominant 7th Chords in 2nd Inversion: V4/3
Dominant 7th Chords
Passing Chords in Music: Analysis
Supertonic 7th Chords
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