You could perhaps write a feature-bare clone. Here's some basic features:
- search (the various search pages)
- trending
- channels
- subscription
- video uploads
- history
- comments
- likes
- upload
- video editing (most of this is in browser, but still)
- livestream
- video analytics
- payment and ad management
- video comment review and moderation
- translation
- captioning
- video replication, multiscaling, caching, and resolution management based on network speed
- video recommendation
- cards, overlays, annotations, etc.
- music and sound effect search
- antispam
- dmca and copyright tooling
And that was 5 minutes of poking around.