The extension works in devtool environment, so it can read the resources information. You will need to restart your browser or re-open developer tool panel, and switch to the Resource Saver tab.
All resources will be saved in "Downloads / All Resources" folder with specific domain name folder.
Tick "Download from all domains ..." to download all resources from all domains.
If "Compress all assets ... (Recommended)" is ticked, all resources will be added into one compressed file (<DOMAIN>.zip).
If you want to manually save the file, enable option from Chrome > Settings > Downloads > Ask where to save each file before downloading