most vitamin D3 (cholecalciferol) capsules are semisynthetic

it is extracted from lanolin (sheep's wool). The lanolin contains 7-dehydrocholesterol, which is then exposed to UV light to convert it into Vitamin D3 (cholecalciferol)

Some "vegan" Vitamin D2 (ergocalciferol) supplements are produced by exposing lichens to ultraviolet light.

humans can store vitamin d in adipose tissue (fat soluble) for indefinite periods of time.

The more time you get in the sun during the summertime the less likely you are to need any vitamin d in the winter regardless of where you live on the planet.