how to create a separate stave in musescore

For a break in the middle of the system, select the bar where the new section starts and do Add > Frames > Insert horizontal frame:

There's an option on the frame to turn off the subsequent brackets/clefs/key sigs but it's all or nothing at the moment, unfortunately.

Then you can hide parts of some staves by right-clicking a bar, going to Measure properties, and controlling which staves you want to be shown using the checkboxes:

If you want to hide staves automatically when they are empty for the whole system, use Style > Score > Hide empty staves within systems (or, if you have nothing selected, Properties > Score appearance > Empty staves).