Try it: press Play. Tap Edit on any card to open its grid, then tap cells to write notes — green fits the chord, amber is in the key, grey is outside it. Build on it fills only the bars you have left empty. Change the key, tap the drum pads, open a demo from the Library. Sound is a few oscillators, not recordings; nothing here is generated, every suggestion is a rule.
The seven looks in the real app are themes you pick in its settings; four of them are here. The Swift build exports MIDI and WAV — this one does not.