Protoctists use any or all of various lifestyle modes. They even change them at different times in their lives. They can be free-living or attached. They can be using sunshine to make food or absorbing nutrients from dead cells. Or they may rely on eating other cells. Like bacteria, they can live outside or inside of other multi-cellular living beings.
Remember the creativity of bacteria led to the evolution of a new cell type, the eukaryote. This seems to have occurred about two billion years ago. Although unicellular, the new cell type was bigger than bacteria, roughly the size of ten bacteria lined up end to end.
Within the cell, every kind of gizmo was created in a rollicking set of experiments in how to live life. The cells also developed an astonishing variety of answers to the question of reproduction.
The trouble with many protoctists is that they have a big impact on the world but they are unicells and impossible to see without a microscope. And you need to be quite skilled to get a good look at them.