When upgrading from 2010 to 2013, we can create a web application and attach it to the content database, do we need to run the command mount-spcontentdatabase, or just attach the database to the web application when we create the web application?
Yes, and no...
To upgrade from SharePoint 2010 Products to SharePoint 2013, you use the database-attach method to upgrade. In the database-attach method, you first create and configure a SharePoint 2013 farm. Then you copy the content and service application databases from the SharePoint 2010 Products farm, and then attach and upgrade the databases. This upgrades the data to the new version. Site owners can then upgrade individual site collections.