How are you supposed to clean the plunger or toilet brush after using?

Stealthly swap it with a new one in Walmart.

You can a brush to clean the old brush, then another brush to clean the second brush and so on.

Or.......just buy a new brush.

I clean my one with dental floss or if needs be piano wire.

I just flush the toilet again and swoosh it around in the running water.

They hardly ever get replaced either. Valid question that I have no answers for.

Take them outside and use the garden hose. Works like a charm. I also have a suggestion for storing the Plumber's Best Friend :D. It keeps your floor clean where it sits.

Take that jug of milk you or your family just emptied, rinse it until it's clean, cut out the bottom so it makes a little "tray," and place it facing up under your plunger. Works like a charm.

This doesn't apply to the brushes because they always come with a holder.

I usually just lick it clean.

After I flush the toilet I just swirl the toilet brush in the fresh water.