In Play mode, for the instrument in question, click on the little > arrow just before it’s name on the left - that’ll expand it out to a piano roll-style editor.
You can then expand those areas if you need to by dragging the area between players down, and also ‘zoom in’ by clicking in the ruler at the top of the page and drag down, or shift click the ruler left/right to move the timeline along.
(Two finger zoom on a trackpad also works.)