This is Shield and Spire, his real name is Zachary Huff and he is a sound engineer and classically trained composer.

This is what he had to say about making the song

The harmony in all of the sections I would be incapable of writing the way I did unless I had the extensive knowledge of tonal harmony which I have. Harmony is my strongest element in music, I use a lot of substitute chords which lead to new harmonic regions. The reason the piece feels the way it does is due to the mild harmonic instability throughout. I am constantly leading into new harmonic territories while still remaining subordinate to the original keys for each part. Some of this harmony is just meer secondary dominant stuff, but some other parts are treated as modal changes which lead into new areas.
I also take advantage of super typical harmonic progressions and then drastically change them and rip them through new tonicized regions. An example would be the big part at 1:58. The harmony begins as a simple i – iv – VI, (or atleast I think it does if I recall correctly, there is a lot of harmonic stuff going on in this piece so I could be misspeaking), but instead of leading to a V chord, it leads to a v chord, suggesting it possibly being interpreted as ii – V – i, in the minor subdominant, D# minor. Also here I can give more specifics if necessary, if you are interested in seeing the harmonic frameworks of the piece.
I think another big aspect that has been influenced by my classical training is the overall attentiveness to each element of music. My melodies are informed by the harmony, and as a result I use the harmony to create a sense of motion throughout the piece.