The multiple camera modules in a S21 have more proprietary features so it's more a matter where they also require a unique and also proprietary Camera app to take advantage of those advanced options. So the Samsung Camera app (software) on your S21 is more tightly integrated to those camera units (hardware) than a more typical set up in most other phones where the camera module is commonly available (used by multiple manufacturers) and a more or less generic camera app.
So yeah, you can install and use third-party camera apps but they can only access the basic feature sets in those higher-end camera modules. All that said, if you go into the Settings menu of your phone's Camera app, there should be options to allow you use manual adjustments. Look in the 'Pro' options