It’s a custom option for the demo only to allow users to visit the most important features of the XStore theme. It’s not some plugin and it’s not included in the XStore theme.
But if you want to implement some floating buttons on your site you can take a look at third-party plugins for these purposes, for example, https://wordpress.org/plugins/buttonizer-multifunctional-button/ or similar.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.