I'm setting up Umbraco 15 and plan to configure it to run multiple sites. (https://docs.umbraco.com/umbraco-cms/tutorials/multisite-setup) Do I need to create a different IIS site for each domain seen in Umbraco?

No, for IIS, it will be just one site. You need to add the different domains for all your umbraco sites to that one site in IIS. So from the perspective from IIS you only have one site with multiple domains attached to it. Settings the cultures and hostnames tells Umbraco what content to serve on what domain.

Thank you!