Microscopic vs. Multi-cellular Life

The video compares a microscopic amoeba, a single-celled organism, to a bird, which is multicellular. It explains that multicellular organisms, like flowers, humans, and marine iguanas, consist of many different cells. The video also suggests that the first multicellular life forms were microscopic and lived in marine environments.

How do multicellular organisms differ from unicellular organisms?

What is likely about the first multicellular life forms?