I can’t zoom in when watching videos on Facebook and YouTube

I don't think it's possbile to zoom in watching videos.

All you can do is to rotate your device and watch them full screen.

Regards

Giulio

Zooming is limited by default on internet videos. In some instances, you can double tap to "fill" the screen, removing all or most of the black bars.

Another option is to go to Settings> General> Accessibility> Zoom. This feature will allow you to manually zoom your screen at almost anytime on your iPhone.