2. Upload or drag and drop the .sppkg to the App Catalog.
3. This will deploy the client-side solution package. Since this is a full trust client-side solution, SharePoint will display a dialog and ask you to trust the client-side solution to deploy.
4. Choose Deploy.
If the check mark "Make this solution available to all sites in the organization" is set when deploying the package, it is no longer necessary to add the app per SiteCollection. In that case, the webparts will already be available. Remember to skip the following step!