The engine of jazz is the ii–V–I, and the notes that make it move are the guide tones — the 3rd and 7th of each chord, which resolve down by step from one chord to the next. Because a dominant's guide tones are a tritone apart, another dominant a tritone away shares them: that's the tritone substitution. Beyond the ii–V–I, jazz colours a key with secondary dominants (tonicizing any chord) and modal interchange (borrowing from the parallel minor). Explore a ii–V–I in any key, hear the sub, tonicize each chord, borrow from minor, and look up the scale to blow over anything.
Any diatonic chord can be tonicized — preceded by its own V7. These are the applied dominants in C; each pulls to its target like a V–I. Play one to hear the tonicization.
Chords borrowed from the parallel minor darken a major key: the minor iv, the ♭VII, ♭VI, ♭III, and the iiø7. Play each, then hear them in a progression.
Type any chord symbol — Cmaj9, F7♯11, Bm7♭5, A♭13, G7alt-ish (use b/#).