And all that stuff? Not diegetic. Where the camera person is standing? Not diegetic. The props people who build the set and put stuff in it? Not diegetic.
So, what's your point?
Good grief. The definition of diegetic isn't rocket science. For something to be diegetic it must exist for BOTH the audience and the in world characters. That's what the term means. So, when the character turns off the radio and the music stops for both the characters and the audience, that would be an excellent example of diegetic. Note, most music in movies is NOT diegetic. After all, when the killer's theme music starts to play in a horror movie, it's not like that music is diegetic. But, when the little girls start singing the Freddie song and the characters in the movie can hear it, that WOULD be diegetic.
So, again, how can something be diegetic without the audience being able to see how results were achieved? If the music suddenly stopped for no reason, that wouldn't be diegetic. That music only exists for the audience, not the characters in the movie.
@AlViking is apparently unable or unwilling to bother to learn what the term actually means and it is causing a break down in conversation.
A diegetic mechanic would look something like this. The characters come across a puzzle - maybe a code on a wall.

The DM hands the players a handout that shows the code. The players try to decipher that code. But, the players aren't actually their characters who, frankly, are often significantly smarter than the players - sorry, not many of us have 18 or 20 Intelligences after all. So, the DM allows the player to make a couple of rolls based on their characters to give hints or possibly extra information about the code that helps the players to solve the code.
That would be an example of using diegetic mechanics. A non-diegetic way would be for the player to simply make a check and get the answer to the code. How did the character break the code? We don't really know. We just know that the character did. The character went from unable to read the code to being able to read the code without any real explanation or idea about how they did that.
Removing traps is often done this way. The PC finds a trap, the player rolls to disarm the trap and the trap is disarmed. How is the trap disarmed? No idea. Doesn't matter. We just know that the trap is now disarmed and no longer a danger. It's not diegetic at all. It's 100% in game world and opaque to the players.