There are some apps that have pre-defined default settings on Android, for example- Browser, Home, Phone and others as shown in the screenshot below. You can select one the available apps as default for the app categories in the screenshot by going to Settings–>Apps & Notifications–>Advanced–>Default apps.
But there are other app categories too like Camera which are not listed in Default apps options, that may want you to choose default once you have installed one extra app than the native one. For example if you have installed Google Camera app you will see below option every time you try to launch camera by double pressing on the power button.
Now, if you select “Always” while opening Google Camera app, it will be set by system as the default camera app while using double press to launch camera. In case you want to clear this default here is what you need to do.
Go to Settings–>Apps & Notifications–>Google Camera or any other default app–>Open by default
Now, tap on clear defaults and the app will no more be the default app and you can set any other app as default.