There are programs which can download videos from YouTube to .mp4 if you paste a link to a video (or playlist). I use a free program called 4k Video Downloader, it works for that purpose. I don’t know if it’s available for Mac.
There are browser add-ons that can do the same thing for some browsers, but I use Chrome and Google doesn’t allow add-ons which download from YouTube.
Once you have the file on your hard drive it’s easy to open with Transcribe!