In photography, long exposure refers to a technique whereby a photo is taken using a slow shutter setting with the purpose of gathering more light amount around the camera censor. This technique results in a photo with a soft effect and is widely used to take photos of waterfalls and Milky Way. Although iPhone (just like other smartphones) has no setting to set the shutter speed manually, you can create a long exposure photo with it. iPhone has a built-in feature to take a long exposure photo.
To take a long exposure photo using iPhone, first, launch Camera on your iPhone. Before taking a photo, make sure Live Location is enabled.

Point the camera to an object you want to take a photo of and press the shutter button.

Once the photo is taken, open it by tapping on the photo on the bottom-left corner of your screen.

Once the photo opens, swipe it up. This will open several effects you can apply. Since you want to create a long exposure photo, you can select the Long Exposure effect.

Wait a second until the effect is applied to your photo and enjoy the result. Please note that the example above was taken on iPhone 12 running iOS 14.4.2
The Bottom Line
Long exposure is a photography technique whereby a photo is taken using a slow shutter setting. Since iPhone has no feature to manually control the shutter speed, there is no option to create a long exposure photo. What you can do is to apply long exposure effect. iPhone has a built-in long exposure effect you can make sure of so that you don’t need to install a third-party app. Never expect much about the result. But if you are curious about what a long exposure photo taken with an iPhone looks like, you can try it.