iOS: https://apps.apple.com/app/midi-drum/id6475945376
Android: https://play.google.com/store/apps/details?id=com.instachord.mididrum
The timeline is shown horizontally.
A “beat” represents each section with grey and black backgrounds.
Each beat includes small circles.
These circles are “steps”.
Each rows are called “tracks”.
Each tracks are assigned with 1 drum instrument.
Left column displays the name of each instrument.