Hi @Tanmoy Das ,
Are you migrating from one tenant in SharePoint Online to another?If it is not migrated from SharePoint Server to SPO,it can only be done by saving as a template or using a third-party tool at persent.
Saving a SharePoint site as a template requires that it does not exceed the default 50 MB site template limit. This could be useful to provide sample content for users to get started with.
In general,retention settings from retention labels travel with the content if it's moved to a different location within your Microsoft 365 tenant. If an item inherits retention settings from the container specified in the retention policy, the retention settings are not moved with the content in the new location.
For more information, you could refer to: Save, download, and upload a SharePoint site as a template ; Retention policies and retention labels
If you plan to use a third-party tool for migration, then I suggest you go to the third-party tool's forum and ask for better help. Thank you for your understanding!
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