Experience Builder and site templates let you create, brand, and publish a custom site that looks great on any mobile device! Choose a template to quickly start your site, and then style the pages to match your company’s branding.
When you create an Experience Cloud site, you can create an Experience Builder or a Salesforce Tabs + Visualforce site.
Reference Article – https://help.salesforce.com/articleView?id=sf.networks_sites_which.htm&type=5
To change template for an existing Community, check – https://www.infallibletechie.com/2021/04/how-to-change-template-for-experience.html.