Windows 10 Mobile camera interface is regarded as one of the best camera interface derived from the Nokia camera app which is now integrated into the OS. So when you open the camera app, the app which opens is the default camera app. Although the camera app offers manual control, you might want to use third party camera apps with more features and also filters.
In Windows 10 Mobile, you can set the default camera app on your phone. This means when you click the camera tile, the default app opens. Here is how to change the default camera app.
Navigate to settings-> Devices and select Default Camera. Here you can choose the default camera app for starting the camera. Some of the camera apps might not be updated to support this feature in Windows 10 Mobile, so even if your favorite app is installed, you might not be able to set it as default.
Once you have set your favorite camera app as default, click on the camera tile on start menu and you can start using it. Below is a quick video