Basically serverside you have a simple route app that is able to run the web applications through a headless chrome instance. The output is cached in static html files. Now when the initial site is served to the end users browser from then on all interactions are all dynamic. This setup works for all kinds of stacks. Angular, vue, react or just custom. The logic is pretty simple. No bloated layers of complexity.

Interesting, thanks very much.

To clarify, the headless chrome is because it's ultimately a js-rendered app, so you just pre-render all the routes on the server to get the final html, cache that, then serve the html on any first request. Then when the app is loaded in the browser, it just does everything client-side?

Indeed. Thats how it works. Of course when your website is pure html you can simply return the html content. No need to go through a headless browser.