The navigation and index sources are built-in types that scan your content directories automatically. You do not need separate metadata files — SSI extracts titles from headings, computes positions from the sorted file list, and generates URLs from your destination path.
In SSI, a page step only sees include steps listed after it in the configuration file. This means navigation and index steps must come after your page steps so the source files are registered before position and metadata are computed.
Include sources provide content for token substitution at build time. SSI looks for emoji+key patterns in your page templates and replaces them with content from the corresponding source.