How can I set a system app (my custom system app) to be NOT DISABLED?
Just like the Launcher or Settings that the “Disable” button is not enabled. So the user can’t disable my system app.
Below is the Launcher’s effect:

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It’s not possible for a third party application (like yours) to define itself as not being possible to disable- It’s important that the user be able to disable any application on their system. If you find a way, that will be considered a security flaw, and it’ll likely be disabled in the next release of Android 🙂
Launcher and Settings are special cases in terms of not being possible to disable, since disabling those would make it difficult/impossible for the user to maintain their own device (or to undo their decision, since they wouldn’t be able to launch the settings Activity and re-enable it).