With the iPhone 11, iPhone 11 Pro and iPhone 11 Pro Max, Apple changed the way the Camera app’s shutter button works in ways that could cause confusion. Tapping it once still takes a single still photo but if you press and hold on the shutter button it now captures a quick video. (Previously, pressing and holding on the button took photos in burst mode. (On the iPhone 11 models, slide the shutter button to the left for burst.)
Once you’ve started taking a quick video, slide your finger to the right to lock recording so you don’t have to keep holding the button down. Tap the white shutter button to take a still image while recording; tap the red record button to stop recording.
For even easier quick video recording, press and hold either of the volume buttons - a single press still takes a photo. Note that quick videos always record with mono sound and at a resolution of 1920-by-1440, for stereo sound and the resolution set in Settings > Camera, use the Camera app’s Video mode.