In ArcGIS Pro 2.4.x and prior if you place a map template at C:\Program Files\ArcGIS\Pro\Resources\ProjectTemplates it was shown in the list of default templates on ArcGIS Pro startup (project selection screen). This was used by organizations to publish organizational standard base map templates to users with no customization of ArcGIS Pro itself. One only had to copy the template to the correct folder for it to show in the list.