is this a Rails thing? Is there a server component?

The branding is incredibly confusing, but: Hotwire (https://hotwired.dev/) is a set of ideas about how to build dynamic web UI (“HTML over the wire”), and the Turbo, Stimulus and Native frameworks are complementary implementations of those ideas in JavaScript and native mobile code. You can use all, some or none of them to build a Hotwire-style app.

The three frameworks originated in Rails apps, so they have good Rails integrations, but there’s nothing Rails-specific about them and you can use them in any environment where HTML is sent from server to client, even a static web site.

Basecamp blog had a write up about this technique years ago. The idea is each app screen is a standard web view. However, it uses a JavaScript bridge to do native page navigation. This provides native swipe back to navigate, as well as the standard animations.

We use it with PHP/Laravel on the server side. It just needs HTML pages and the JavaScript of Hotwire Turbo in that page.

For the fancy streams and frames you need some extra headers being sent from the server, but also that is doable from PHP.

It works great with Rails but can be used with any other framework or without.

You need some kind of server to render your website but it's all built on top of HTML