What part of marijuana is used to fill capsules?

Generally speaking, the compound THC is the part of marijuana that is put in capsules.

One way to accomplish this, is to extract the THC from the plant buds, and infuse it into an oil substance. You can do this by simply heating the buds in oil on a stove-top or slow cooking crockpot. This oil substance can then be directly added into the capsules, or absorbed into a dry substance (corn starch or baking powder), and the resulting gel substance added to capsules.