One cell, or many
When the cell theory says that all living things are made of one cell or of cells, that "or" is not an accident. It divides living things into two groups.
Unicellular organisms
Organisms made of a single cell are called unicellular. That one cell does everything living requires.
Multicellular organisms
Organisms made of many cells are called multicellular. Here the different jobs are shared out among separate cells.
How much does that change?
A unicellular organism has to carry out feeding, respiration, excretion and reproduction all inside the one cell. In a multicellular organism those jobs are separated, so a group of cells can specialise in just one of them — which is what the next page is about.
The unit does not change
Whether an organism has one cell or millions, the unit is the same. A large organism is not one with larger cells but one with more of them.