One thing to notice is that the elements of music, rhythm, melody, and harmony, are based on relationships between periodic impulses. E.g. drum patterns, vibrating strings, musical sequences, and melodic intervals.

Here is a small yet information-dense book:
https://woodenbooks.com/index.php?id_cms=8&controller=cms#!M...

Keeping the fundamental unity of musical elements in mind, build a framework of knowledge and practice including the following concepts.

Harmonic series as the basis of musical intervals
https://www.oberton.org/en/overtone-singing/harmonic-series/

Intervals as ratios, and how the simplicity of ratios relates to consonance:
- https://en.wikipedia.org/wiki/Interval_ratio
- http://legacy.earlham.edu/~tobeyfo/musictheory/Book1/FFH1_CH...
- http://hyperphysics.phy-astr.gsu.edu/hbase/Music/mussca.html

Polyrhythms as simple intervals (3:2, 2:3, 3:4, 5:4, etc.)
- https://en.wikipedia.org/wiki/Polyrhythm
- https://www.musical-u.com/learn/making-sense-of-polyrhythms/
- https://mynoise.net/NoiseMachines/polyrhythmBeatGenerator.ph...

Ear training to recognize intervals and notes
- https://en.wikibooks.org/wiki/Music_Theory/Scales_and_Interv...
- https://tonedear.com/ear-training/intervals

Building chords from intervals
- https://en.wikipedia.org/wiki/Interval_(music)#Intervals_in_...
- http://www.thejazzpianosite.com/jazz-piano-lessons/the-basic...
- https://tonedear.com/ear-training/chord-identification

Building major key from intervals (R, M2, M3, P4, P5, M6, M7)
- https://www.howmusicworks.org/202/The-Major-Scale/Intervals-...
- http://onlineguitarlessons.co.uk/major-scale-intervals-and-t...
- https://tonedear.com/ear-training/scale-identification

Stacking chords from scale tones
- http://www.jazclass.aust.com/scales/scastc.htm

Extended chords to add color and emotion
- http://www.simplifyingtheory.com/extended-chords/
- https://en.wikipedia.org/wiki/Extended_chord

Major key chord function
- http://hubguitar.com/music-theory/chord-function
- http://openmusictheory.com/harmonicFunctions.html
- https://www.jazzadvice.com/chord-function/

Circle of KEYS as a compositional tool showing diatonic chords for each key and common modulations
- https://harmoniousapp.net/p/d9/Circle-of-Fifths-Keys
- https://www.classicfm.com/discover-music/music-theory/what-i...
- https://blog.landr.com/circle-of-fifths-infographic/

Other geometric representations of musical elements
- https://en.wikipedia.org/wiki/Tonnetz
- https://imaginary.github.io/web-hexachord/
- https://en.wikipedia.org/wiki/Euclidean_rhythm
- http://www.groovemechanics.com/euclid/
- https://dev.to/erwald/euclidean-rhythms-and-haskell-5ecj

Rotating major scale to get modes (and how diatonic chords relate to modes)
- https://blog.landr.com/music-modes/
- http://www.jazclass.aust.com/improvisation/im12.htm
- https://onlineguitarbooks.com/2012/01/06/functional-harmony-...

Chord substitution (functional, modal, and tritone)
- https://lotusmusic.com/lm_chordsub.html
- http://www.simplifyingtheory.com/chord-substitution/
- https://mixedinkey.com/captain-plugins/wiki/easy-chord-subst...
- https://en.wikipedia.org/wiki/Borrowed_chord
- http://www.simplifyingtheory.com/borrowed-chords/
- https://en.wikipedia.org/wiki/Tritone_substitution
- https://jenslarsen.nl/tritone-substitution/

Modulation
- https://en.wikipedia.org/wiki/Modulation_(music)
- https://www.musicnotes.com/now/tips/a-complete-guide-to-musi...
- https://www.artofcomposing.com/the-art-of-modulation-part-1

Compositional forms
- https://en.wikipedia.org/wiki/Musical_form
- https://courses.lumenlearning.com/musicappreciation_with_the...

Some excellent musicians/teachers
- Jacob Collier
- https://www.imusic-school.com/en/music-theory/lessons/jacob-...
- http://brightonjazzschool.com/jacob-collier-masterclass
- https://www.youtube.com/user/jacobcolliermusic/playlists
- Kate and Ray Harmony
- https://hackmusictheory.com/home
- https://www.youtube.com/channel/UCDKiHSPstsj0silp519gt6w
- Nahre Sol
- https://www.youtube.com/channel/UC8R8FRt1KcPiR-rtAflXmeg
- Jazz Duets
- https://www.youtube.com/channel/UCqimxUbWsE26KSpx2_OcmmA
- Signals Music Studio
- https://www.youtube.com/channel/UCRDDHLvQb8HjE2r7_ZuNtWA
- David Bruce
- https://www.youtube.com/user/davidbrucedotnet
- Benn Jordan
- https://www.youtube.com/user/angeldvst
- Omri Cohen
- https://www.youtube.com/channel/UCuWKHSHTHMV_nVSeNH4gYAg