To lock your manual focus and exposure settings for upcoming shots, touch and hold the focus area until you see AE/AF Lock; tap the screen to unlock settings.
On iPhone 11 and later, you can precisely set and lock the exposure for upcoming shots. Tap the Camera Controls button, tap the Exposure button, then move the slider to adjust the exposure. The exposure locks until the next time you open Camera. To preserve the exposure control so it’s not reset when you open Camera, go to Settings > Camera > Preserve Settings, then turn on Exposure Adjustment.