As soon as you click the Use the template button, you'll be immediately taken to Softr studio and can start working on the template. If you don't have a Softr account, you'll be prompted to register first (a Free account will work just fine).
All the Softr templates are completely free of charge and can be used with any of the Softr subscription plans.
No, unfortunately that's not possible. You'd need to create a new application using the template.