Nice firepit in decking project.
The important thing here is to plan ahead. Notice how he’s marked out the position of the firepit in relation to the seating. Although the decking was built first, the firepit itself is built from the ground up.
The one slight issue, is lack of ventilation. Those tiny pipes in the base and nowhere near enough. You need 100 square cms. However he’s never reported a problem so it must be ok.