What is the difference between the structure and vital processes of cells in a multicellular and unicellular

What is the difference between the structure and vital processes of cells in a multicellular and unicellular organism? What multicellular organisms were among the first to appear on the planet?

A single-celled organism is a cell that is at the same time a whole organism that breathes, feeds, moves, grows, develops and multiplies. For a multicellular organism, these functions are also characteristic, but very often these functions are divided between groups or individual cells.
The oldest unicellular organisms were algae, among plants, and sponges, among animals.