There multiple ways to create/add Projects in SI 2018 within the Project Explorer.
The New->From Template function allows you to create a project based off of a saved template. Click here for details.
To create a Template, simply select an existing Project in the Project Explorer and click the [Template] button:
You will be prompted to name the Template as well as add a description. You can choose whether or not to import any additional files like Visio or AutoCAD drawings:
The New->Import Project function allows you to import legacy SI5.5 Projects into SI 2018. You can import SI 2018 Projects here as well, either ones you've previously exported or Projects you receive from other SI 2018 users outside of your company. This is also how you can import Sample Projects. Click here for details.
This allows you to to create a Project from a Mobile Quote. This will open a dialog prompting you to choose a Quote that has been synced from Mobile Quote to SI.
Click here for details.
This allows you to to create a Project from a Third-Party Integration with SI. Click here for details.