We got the client requirement of migration. They have SharePoint online O365 site which needs to migrate on SharePoint Foundation 2013 (SharePoint Foundation + SQL Express). As we know we cannot do backup of SharePoint online, how can do the migration (backup and restore)?
The internal versions of SharePoint Online and SharePoint 2013 on premises are fundamentally different. The SharePoint 2013 has a major version of 15.0, whereas Office 365 environment is currently running at the major version of 16.0. Due to this major difference, you may not get the content intact (with full fidelity) in SharePoint 2013. So, backup and restore may not be the best option.
My suggestion is to go for granular migration with third-party SharePoint migration tools such as Vyapin's Dockit Migrator for SharePoint. You can get the intact results when performing migration from SharePoint Online to On-premise, while leveraging all the functionalities of Dockit.