Since it is from a living thing, it is of cells. The rectangles that you see are really the plant cells. These cells like warm, moist places that have a source for food. When they are in such a place, they reproduce, sometimes by budding. When the bud gets big enough it will break away to make buds of its own.
Yeast makes carbon dioxide.
When we put the yeast in bread dough, the yeast eats the sugar in the dough and the gas makes the bread rise. On the lowest objective, put the pointer between the darker pink and the lighter pink cells. Switch to a higher objective and focus. The darker pink cells are layered, one on top of another. The lighter pink cells are more round.