According to [1], the downloaded video is AES-encrypted. Decryption key is stored locally in an IndexDB. I tried building a Python script using plyvel [2], a Python module for reading IndexDB/LevelDB, some time ago but IIRC, it had trouble reading the Chrome DB. Maybe Google's dfindexeddb [3] works. I didn't try it, however.

[1] https://github.com/Marsel-marsel/youtube-premium-video-downl...

[2] https://github.com/wbolster/plyvel

[3] https://github.com/google/dfindexeddb