The things that something is composed of are its parts or members. The separate things that compose something are the parts or members that form it.

When someone composes a piece of music, they write it.

If you compose something such as a letter, poem, or speech, you write it, often using a lot of concentration or skill.

If you compose a picture or image, you arrange it in an attractive and artistic way.

If you compose yourself or if you compose your features, you succeed in becoming calm after you have been angry, excited, or upset.