Song structures - AABBAABBCCAABBCCC etc.
Mixing - eq for height, volume for proximity, panning for.. panning, effects, mastering.
Software - Rezound, Aqualung, Milkytracker, Qtractor, Phasex, Yoshimi, Jamin Remix old and recent stuff - add layers, take loops, master.
Use own sounds - created in Rezound from scratch (generate noise etc), found sounds, old sample collections.
Also, not just song structures but chord sequences and harmony too. And melodies, counterpoint, syncopation, time signatures.
And there's music programming - Pure Data, Fluxus, Csound, etc.