This extension is simple yet useful, or it was until the last update. By default, it went to loader.to to download. So if you want it to work the way it did, just go to the extension settings and set the mp3 option to:
https://loader.to/?link={url}&f=1&s=1&e=1&r=coconvert
and the mp4 option to:
https://loader.to/?link={url}&f=7&s=1&e=1&r=coconvert
This format is simple: the part f=1 tells it what kind of file to download. So you can experiment with different numbers to give different default downloads for audio and video, or simply choose an option once you're directed to the download site. I have chosen f=1 which is mp3 audio files and f=7 which is 1080p mp4 video files. If you want bigger than 1080p, you can use f=8 for mp4 1440p, f=9 for 4k (4k and above uses webm files instead of mp4 to save space it look like) and f=10 for 8k.
I also know that s=1&e=1 control the default playlist downloading, so changing that will change how it downloads playlists.