Where does vitamin d in the supplements come from nowadays? i think it used to come from cod liver oil, right?

Natural sources: Vit d is produced industrially from natural sources such as wool-oil and yeast and mushrooms. Often a precursor is obtained from these sources and then exposed to uv light to be converted to vit d, similar to how it is made in our skin.

Vitamin D: There are two forms of vitamin d supplements, d2 and d3. D3 is the form naturally made in humans by exposure of the skin to uv, and more potent than d2. D2 is produced industrially from yeast or mushrooms. D3 is produced industrially from its precursor substance, dehydrocholesterol, in wool grease (lanolin) from sheep or other woolly animals. Exposure to uvb light and purification are needed.