1.If you need to close SPMT before a submitted job has completed, you can restart the tool from any computer.
Note: To resume a submitted migration job, it has to have been running at least 5 minutes. It pauses at the point you closed the SPMT. If your submitted job was running less than five minutes before the tool closed, you must resubmit the job.
To resume migration jobs
(1)Launch SPMT. Using the same Microsoft 365 username and password you used when you originally submitted the job, select Sign In.
(2)After you sign in, a screen displays any paused migrations, providing details about what has been completed and what remains.
(3)If you want to add additional migration tasks, select Select new sources and destinations; otherwise, select Next. Your migration jobs will be resumed. If you are migrating files from an on-premises SharePoint Server, you are prompted for your username and password for that site.
2.During migration, files will be stored in the SharePoint Online cloud space.
Reference:
https://docs.microsoft.com/en-us/sharepointmigration/how-to-use-the-sharepoint-migration-tool