One interesting feature hiding inside Apple's stock Camera app is a level that helps you take photos of documents and nicely plated food from a top-down perspective. Unfortunately, you'd never know it was there since it's not enabled by default and there is no obvious setting for it.
To unlock this hidden feature on your iPhone, you need to be running either iOS 11, iOS 12, or a newer update. Good? Head to your Settings app, then tap on "Camera" to view its settings. The Camera app used to be bundled in with the Photos app's settings in iOS 10 and earlier, so it's nice that they finally separated them.
In the Camera settings, all you have to do is toggle on "Grid." This will overlay a grid with four lines making 9 equal boxes over the viewfinder in the Camera app, which helps you apply the "rule of thirds" better in your photographs for more balanced compositions.
By enabling the "Grid" feature, you'll also be enabling the new level, which shows up as a set of thin white and yellow crosshairs that turn into one bold yellow one when your iPhone is perfectly parallel with the ground or ceiling. While the grid shows up in all shooting modes except Pano, the level only works in Photo, Time-Lapse, Portrait, and Square modes, not in the video or panoramic options.
These crosshairs help you take more balanced photos from an overhead view without an expensive tripod arm/mount, which is handy for document scanning, food pics, and pretty much anything else where'd you like to get a straight down bird's eye view of your subject. This is especially helpful for all your overhead knolling images, where items are placed at 90-degree angles from each other for a clean, symmetrical-looking vibe.
It also works well for down-top sky or ceiling views, where you're laying on your back and taking photos upward. Hopefully, your skyward pics look better than my examples below.
Maybe one day Apple will also include this crosshairs-based level for straight on wall views or even portrait photos, where the iPhone would be perpendicular (at a perfect 90-degree angle) to the ground and parallel to vertical walls.