Assuming no mods, monsters do not scale by player progression. Any given monster will always have the same stats.

They do scale by number of players in the game, so they will inflict and endure more damage as more players join. It is less than 100%, so if every player is actually skilled and prepared, they will outmatch enemies.

Creatures can spawn with one or two stars. These are more powerful creatures with potential for more loot drops. This is not a scaling mechanic.