In the app, Activity tells you which work changed the file and when: see How do I find out who last changed a specific file?.
Ask the agent to read the version you want first: reading a stored version puts nothing live. The restore is itself a write: it checks the stored content against its recorded size and hash, checks the syntax of a .php file, and keeps what it replaces, so a wrong restore can be undone the same way.
There are limits: at most 20 versions per file, none over 2 MB, never binary or media files, and for files that look like configuration only the fact that they changed is recorded, not their content. Moves keep nothing (a copy keeps the file it overwrites, an append or a number bump the content before it), and a delete keeps nothing unless the agent asks delete_path to keep the last version of each deleted file (keep_version, since version 13.2.49.79), versions follow the path (a renamed file starts over), and they live on the SyncNow server, not in your site's backup.