Good day to you SuToukhy,
It sounds like you're noting the Camera app preview is darker than another device and that there are some performance issues and a third-party app loading issue. In attempting to troubleshooting these issues, it also looks like you reviewed See your purchase history for the App Store, iTunes Store, and more
Let’s begin by ruling out settings differences and environmental issues for the display brightness. First, if you have a screen protector and/or case on either iPhone, you’ll want to remove that on both, so they are running the same. When you change the brightness in Control Center, do you notice a change? Also, check to be sure the color temperature and such are the same on both models: Adjust the brightness and color temperature on your iPhone, iPad, or iPod touch
As for the performance issues where the device is unresponsive, please be sure we’re working with iOS 14.6. You can verify that in Settings > General > About. If not, let’s get a backup made and that update installed.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Next, check the App Store app and make sure all app updates are installed. Then for app specific issues like with WhatsApp, you follow the steps here: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open
Let us know what happens.