This workshop from JSNation conference in June of 2020 provides a detailed introduction on how to create a Frontity theme.
The above video includes the entire 3 hour workshop.
This github repo contains the detailed contents of the workshop as well as the example code: https://github.com/frontity-demos/2020-06-jsnation-workshop
Addendum - Connecting to Gutenberg
This detailed post references a conference presentation discussing how the Frontity team made their theme play well with the WordPress block editor: https://frontity.org/blog/connecting-gutenberg-and-frontity/