What are those white spots which found at the bottom of the pot unit sometimes? How can I remove them? How to avoid them?

These spots are water marks or water scales, and you can remove them by using small amount of white vinegar.

The cause of these marks is:
The pot interior may still have small amount of water left while the pot is still hot. It will then be dried up by the remaining warmth, and form water scales.

To avoid them, you can fill in the pot with certain amount water and soak the pot.