Most of the self watering pots available in the market, are using the capillary action of water. They are fairly expensive and cannot be easily made at home.
There is a lower compartment in each pot for storing the water and a tube for connecting the lower water filled compartment to the upper compartment of the pot which contains soil, mulch and a plant
The empty plastic bottle method is fairly cost effective as the waste bottles are recycled. However this method is mainly suitable for large size pots and time is required to experiment, so that water is released slowly
The simplest way of making a self watering pot is by using an empty plastic bottle , making a small hole and filling it with water. The water will slowly leak from the plastic bottle and keep the soil damp, so that plants have enough water required for growth.
However most of the videos or tutorials do not specify the size of the hole to be used and the location of the hole. As a result some of the water is drained, while the remaining is retained in the container/bottles
The amount of water drained depends on the location of the hole, size of the hole, and how the bottle is position, whether it is kept horizontally, vertically or at an angle
On Youtube and other websites also there are many videos on how to make a self watering pot, or ensure that the plants will get water when the home owner or gardener is away from home. many of these methods are fairly effective for a single plant and pot , however if 30-40 plants have to be watered automatically for 7-8 days it can be a challenge for the gardener.
Many of these methods involve recycling water bottles and other bottles which are otherwise discarded as junk