A headless CMS usually but with a strong preference for just hardcoding anything that won't change very often. I'd rather have to deploy the app again than have a dependency on a network service. CMS content is strictly limited to things that need to be updated more quickly than the release pipeline can handle (e.g. marketing content that needs to be updated to react to the response, legal things like T&Cs that need to be live ASAP when there's a change, etc.)

The actual content for the site (descriptions, help content, etc) all comes from a locale-specific JSON file that's exported from a translation app.