All protists are unicellular by definition

Protists are diverse in part due to endosymbiosis, a process by which small, unicellular organisms are engulfed by larger unicellular organisms and eventually evolve to become organelles

A "protist" is broadly recognized as a eukaryote that is not an animal, fungus, or plant

Malaria, red algae, green algae, and dinoflagellates are all examples of protists