As announced by Microsoft some weeks ago, a new option to copy documents between SharePoint Online (SPO) sites is being rolled out. This option will allow to copy the selected file to any of the sites where the user have access:
From a modern SharePoint document library, select the file you want to copy and then do click on the “Copy to” option available on the actions bar or the document actions menu:
Then, you will see a panel where you can choose to copy the document to another location in the current document library, to your personal OneDrive for Business or to any other site where you have access to. Note that information about sites that can be accessed by the user is provided by the Microsoft Graph.