One backup by its exact id, as a JSON:API document of type site-backups: everything the listing shows plus the detail fields, answered by default unless fields[site-backups] narrows them: content (the areas included and the exclusions), restoration_allowed (whether the website could take a restoration from it - a restoration is started from the dashboard only) and has_restoration_in_progress.
include=items adds the parts of the copy as site-backup-items resources (type: database, core, plugins, themes, mu_plugins, content, uploads; status, phase, size, total_items), one row per part, to explain a failure part by part.
Path variable siteBackup is the exact backup id from the listing. A backup of another organization answers 404. A read-only token is enough. The MCP twin is the site-backups-show tool.