A Microsoft file hosting and synchronization service.
Hi Tony,
For the first question, if the users sync all documents in a library to their a local folder, the changes they make on their local copy will update into the cloud automatically. More information can be found in the article: Sync site libraries to your computer.
In addition, if they edit one document at the same time, we can use Co-authoring and Version features on SharePoint Online.
For the second question, when we make changes (remove, add or edit) on the files in the synced local folder , the files will be removed, added or edited in the SharePoint site. Therefore, the files removed by the first user will disappear from all synced users. If some users remove all files accidentally, we may use Recycle Bin in SharePoint online to restore the deleted files. About the Recycle Bin, I would like to share with you some articles: Restore content with the Recycle Bin and Manage the Recycle Bin of a SharePoint Online site collection.
Regards,
Shirley