It is essentially a compressed container (similar to a .zip file) that holds contact details and metadata in a format unique to Samsung.
Converting a .spbm file (Samsung Smart Switch Backup) to .vcf (vCard) is a multi-step process because .spbm is a proprietary, encrypted backup format rather than a direct contact file. 📂 Understanding the .SPBM Format spbm file to vcf link
The most reliable way to convert these files is to restore them to a device first and then export them in a standard format. It is essentially a compressed container (similar to a