Q.Boxwood in pots
I want to plant boxwood shrubs in containers. What should I use in the bottom of the pots under the soil?

You should purchase a good quality pot with good drainage. Check the holes in the bottom of the pot.
Buy a good quality potting soil. Do not use dirt from the garden as this will compact to hard.
I recommend filling the entire container with soil. If the pot will be heavy and need to be moved you can purchase a little pot lift with wheels.
Here are a few links for you with more information.
https://www.gardeningknowhow.com/ornamental/shrubs/shgen/potted-shrubs-growing-shrubs-in-containers.htm
https://www.gardeningknowhow.com/special/containers/container-plant-watering.htm
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/boxwood-care.htm