Why does smoke and hot air go up a chimney?

Disclaimer: This is just a guess.

Since its gas the gravity doesn't push it down it goes through it?

there is a sequence and logical steps to give an answer of a trouble.

1) strategy is necessary to develop an project idea.

2) to study and know the environment where you are going to work ( ecosystem).

3)   the materials resources to built the ideas.

4) to give a priority order in the context that you study. in this case human needs.