Playback Volume for Piano Score

Add a quieter dynamic to the left hand stave and in the inspector change its range to "staff". Also change the range to "staff" of any dynamics applied to the right hand staff. The default range of dynamics applied to a grand staff (two stave piano part) is "instrument", i.e. the dynamic affects both staves equally.

See https://musescore.org/en/handbook/3/dynamics#adjust-range and https://musescore.org/en/handbook/3/hairpins#edit-properties

Instead of using dynamics, I recommend using the Inspector to alter the velocities of the notes themselve.s That way the relative difference is preserved even as you add, remove, or alter dynamics later. It also allows you to get more specific - it's not just so much the LH you want quieter, but the melody you want louder, and that also means, not all RH notes, but just voice 1, just top notes of chords, etc.