Get a WebPage that is modified after loading in swift?

So I want to download a .mp3 file from a Youtube video. But YT only provides a .mp4 file (afaik). So I have to convert the Video to a .mp3 File. But I don't want to download the video and convert it on device, since it takes too much data..

HERE was already mentioned that there is an API for it.. sadly it doesn't work anymore...

What I now found is this: https://youtube7.download/mini.php?id=HY_OIwideLg

But they load the .mp3 link (that I need) asynchronously. So I can't simply do URLSession.shared.dataTask(with:) because I will get the 'loading'-page only, with no link...

So the question is: How can I get the final loaded html site that returns from this php script?