how do I switch between portrait and landscape on iPhone 13?

Hey there ,

See ☞ Use camera modes on your iPhone, iPad, and iPod touch - Apple Support

You mean, rotate your phone?

If your phone is vertical (camera notch at the top), then you're in portrait mode.

If you're holding the phone horizontally (camera notch on the side), you're in landscape mode.

The caveat to this is movie recording - moves will always record in whichever orientation the phone is in when the recording starts. If you rotate the phone mid-record, the phone won't switch modes and your video will be on its side.