We use Backup Content DB using SQL back up most likely for Migrations Option, meaning you are migrating from SharePoint 2016 to SharePoint 2019.
in that case, to migrate we use the command Mount-SPContentDatabase
So most likely this is used for migrations.
But if you use the Backup-SPSite option using PowerShell, then you may be moving your site collection in the same Farm from one web application to another and you can definitely move it from one Farm to another but within the same SharePoint Version, so this is not migration.
So the what do you want to do is …. what you will use..
Hope that helps understand things right.
