Hello,
I had a factory radio unit that I've replaced with a 2DIN android unit, as far as I remember, there's a plastic plug behind the shelf that positions and secures the radio in place (the gray part in the middle):
I assumed that it would be the same for the shelf, but I've found a listing for a 1DIN + shelf and it looks like the shelf is held onto the radio with some kind of hook system (sorry, no more photos):
This radio unit is to be removed by putting U shaped removal tool in the small round holes on the front that will unlock the hooks on the side:
These will not work for the Sony unit you have, I have no idea how is it secured. Might be the same bracket on the back as on the photo above.
There is this foldable document / tablet holder on top of the radio. Unscrew it and you will get access to the back of the radio / shelf unit, so you could see how it is attached.
As far as I remember, I had to cut out a plastic support on the sides, which prevented the 2DIN unit to get in place.
EDIT: another photo of the same 1DIN CD-radio that shows 2 things from a better perspective:
The left arrow shows the original hooking system released by the U shape removal tool. Yours is secured in a different way.
The right arrow shows where the shelf is attached to the radio. I think this part around the radio is attached to the dashboard with those being expanded when the radio is out: