Not the most complicated answer, but I imagine you have some kind of hope for these files. Dont.
The files you delete arent deleted, youre right on that point. The file simply has a switch flipped that makes it ... ohh, lay down like a railroad log.
On the track of progress. If the rail gets laid over it, you cant get the log back. You can see that it existed there, though.
Where you are fuzzy is that something isnt deleted until the drive runs out of space. No.
The file remains exactly where it is at and anything new that you put onto the device will go over that in sequence. If you deleted 10 megs of files and then your next app update installed 60 megs, AND that area was next up to be written, (A logical 'block' of information, SD card/hard drive, whatever you wanna call it.. isnt a random thing where things are just put whereever. In the interest of speed, things are put where they are supposed to be read faster. A chunk of program A should be next to all parts of program A.
Translation: You pissed in your gas tank worth 150 mL of fluid, then you siphoned out that 150 mL. If you did absolutely nothing, you could go in at THAT point and generally detect what was in there, and where and how much. It wouldnt be exact, and you should expect little.
But if you did ANYTHING after that, you already deleted even the trace amounts of that piss.
Its gone. It was gone the very next time you pissed/installed an app, more exactly.
The filename could still be in a file table, somewhere. The record of its existence is technically there, but that would be like manually browsing the bible for a passage that includes the word 'holy'. Needle in a haystack.
Then you could go and look at where the needle used to be, within the haystack. Expecting anything. You could 'read' what occupies that spot, and try to make sense of it, sure.
In the sense that you could carve into a block of cheese to find the perfect sphere at its core. You, or your program.. would have to define what area to lift out and put into a new file.
And then you would have to not only recover to a different drive, you would be subject to whatever deletion your operation system did, and more precisely what kind of format you did to it.
When you think of 'CIA level' and for future reference, its DOD level... you may be thinking of some espionage type affair where the file can be recovered. In that essence, yes. Due to the nature of how a text file works, erasing it and shredding it ONCE might leave over enough bits to 'guess' what was inside. Assuming it was simple text. If it was encrypted, then it already gets thrown into ordered chaos. A 1x shred without encryption key will make it worse than twice as difficult.
Your pics and videos operate on a different principle of 'encoding', and if you recovered 50% of your best video, that thing wouldnt just 'play' and be seen as a fuzzy picture like tuning into the playboy channel on 1980s classic cable. There is no video player that performs that level of error recovery.
THink of the BEST, most high quality movies you download. Every so often you STILL get a decoding error. That error might just be an audible 'squeak' when playing, or the color information could be fucked until the next keyframe. Thats best case scenario. Playing half of a file will, at best, show you the chunks that are playable and not softlock the program into a freeze.
And that is your answer, in simple and medium detail. Its gone, sirs. I'm sorry, Mister Lewis.