Nous nous lavons le soir. Ils se lavent tous les matins. Why use 'le soir' not 'les soirs' in that sentence? What is the difference?

In French, the singular "le soir" already means "generally in the evening". You don't usually use the plural, except if you use an expression like "tous les soirs".

Je me suis lavé les mains ce soir. -- I washed my hands tonight.

Je me lave les mains le soir. -- I wash my hands at evening (meaning generally, all evenings).

Je me lave les mains tous les soirs. -- I wash my hands every evening.