The OnePlus 7 Pro is one of a few Android phones to support a display refresh rate of 90Hz. While this makes for a buttery smooth scrolling experience, using the 90Hz mode also drains the battery much faster than the regular 60Hz rate that most phones have. The 7 Pro does have a dynamic, content-aware switcher built-in, but if you'd like to have more control, the recently released Auto90 app might be for you. It allows you to set preferred display modes on a per-app basis, though the setup is fairly complicated.
First up, Auto90 will only work on a OnePlus 7 Pro running OxygenOS based on Android 9 Pie. Should you be running a custom ROM, you won't be able to use the service. If you match that requirement, the app will start guiding you through its setup process, telling you that you need to enable the write_secure_settings permission via ADB (if you're not rooted). To do that, you have to activate developer mode on your OnePlus 7 Pro, connect it to a computer, and run commands. The developer offers a walk-through on a GitHub page. Additionally, the app uses an accessibility service to reliably detect and switch modes.
Left & Middle: Auto90 guides you through setup. Right: After everything is up and running, the app will offer you a list of all installed apps along with toggles for display modes.
When you're done, Auto90 will greet you with a list of installed apps along with toggles to switch between 90Hz, 60Hz, and OnePlus' auto mode. In the beginning, the service sets up some defaults for you, allowing 90Hz for web browsers, launchers, and social media, while disallowing it for media players, which can't properly take advantage of it in the first place. Should you decide that you want to return to this state after playing around with the toggles, you can simply hit the trash can in the app's top bar.
If you're willing to jump through the setup hoops, you can download Auto90 on the Play Store for $1.50.