Most browser music tools are a desktop multitrack timeline shrunk to fit a phone. The owner looked at the field, liked one incumbent’s clean look, found another’s colours and feel cheap, and asked the question in the pull quote.
The shape
The design that came back drops the timeline altogether. A song is a list of sections, played in order. A section is four cells, one per instrument. A cell holds a pattern: a grid of sixteen steps, which is a thing a thumb can fill in on a phone screen with no zooming and no dragging across a ruler. Arranging a song is reordering sections, not sliding clips.
Phone-first means that the phone layout is the design and the desktop is the same design with more room, not the reverse. The first round of mockups was built on this and accepted.