youtube is taking up all my storage! how do i delete what i've already seen?

Hello Annie1101,

Thanks for reaching out in Apple Support Communities. We understand that you're having trouble with your iPhone, it sounds like the YouTube app is taking up most of the storage. We're happy to help. Go to Settings > General > iPhone Storage, tap the YouTube app, then tap "Offload App." This can help to free up space used by the app.

Check out the following support article:

How to check the storage on your iPhone, iPad, and iPod touch

Cheers!