In the Genesis Flood story, Noah built an altar after leaving the Ark.
Genesis 8:20 says that Noah built an altar to the Lord after the waters had receded and he had come out of the Ark. He then took some clean animals and clean birds and offered burnt offerings on the altar. This is the first action Genesis records Noah taking after the Flood survivors leave the vessel.
The altar episode follows the bird releases and the drying of the ground. It also explains why Genesis earlier distinguishes clean animals from other animals: the clean animals are used in Noah’s offering. The passage does not say that Noah built a permanent temple or city at that moment.
Later biblical stories contain other altars built by figures such as Abraham, Isaac, and Jacob. Those episodes should not be merged with Noah’s altar. In Genesis, Noah’s altar is specifically associated with thanksgiving and worship after the Flood.