For uploading/downloading your project, the only method was to use Mergin web interface. But with this plugin, users can interact with their projects and data directly from within QGIS desktop.
You can install the Mergin plugin from QGIS plugin repository. Once it’s installed, you will see Mergin provider from your QGIS browser panel.
To use the plugin, you need to sign into your Mergin account. You can then easily:
For more information about this plugin, see user documentation.