The conditions micro-organisms need to grow and reproduce on agar plates are food, moisture and warmth. If plates are left open, micro-organisms (eg spores from fungi) can land and grow. We can also use cotton swabs to grow bacteria on the plate. The nutrient agar provides water and food. Placing the plates in an incubator at 25 degrees provides warmth.
Bacteria reproduce quickly by binary fission, producing millions of colonies that are visible as shiny dots due to the slimy capsule around bacteria.
Fungi look furry because of the hyphae. The darker areas are where the fungi are producing sporangia with asexually produced spores. These grow above the surface so the spores can be carried by wind.
Both bacteria and fungi are saprotrophs, so they can digest nutrients on the plate and reproduce.
Viruses can't grow on agar plates as viruses must have a living cell as a host to enable them to carry out reproduction.