In reply to Peter.
A static website is seperate from your WordPress website. WordPress only functions as a data store where you manage your content in this setting, the JAMstack fetches the data through REST (or GraphQL) and builds a completely new website.

This means:
No theme, no CSS nor no JS served from WordPress. No headers, footers, sidebars nor widgets, you only get the data you want/need to build your website.

The JAMstack introduces a bit of complexity WordPress has worked hard to eliminate, but at the same time it’s offering incredible speed and control. For many that’s more hassle than they want to deal with for others it fixes optimization problems they have with the monolithic solution.