The UI Loader is a component in Antora responsible for downloading and caching the UI bundle, then extracting and loading the files from that bundle into a UI catalog.
The UI files are combined with the content files at the end of the Antora documentation pipeline to produce a complete site.
Antora is a modular static site generator designed for creating documentation sites from AsciiDoc documents.
Its site generator aggregates documents from versioned content repositories and processes them using Asciidoctor.