What does the storage data I/O and decryption process look like in a device encryption context when you play a random middle portion of a big movie file (e.g. 50 GB). Does the whole file have to be decrypted first?
I am afraid I understand very little about either encryption or (unencrypted) video file I/O and cannot phrase the question in a highly informed way.
But I imagine that, when I load a 2 hr movie file and take the pointer to the 1:00:00 point, the player app goes directly to the right point, whether by means of some markers or some calculation (based on frame rate etc.). But if the file is encrypted, such markers or other information would be unavailable to the player app and would only become available when the whole file has been decrypted? The player app might request such decryption and get it (before it can start playing), but I suspect this would be inefficient and probably not how it is done. (A small device may not have 50 GB of RAM to hold the decrypted file and may have to write it to storage.) It is possible that the problem that I imagine does not even arise because it is based on a misunderstanding (from my experiences of using things like 7zip to encrypt and decrypt, which should be no model for understanding device encryption).
I do not know if the answer depends on encryption type. I would like an answer that explains generally applicable principles. But if a more specific question is desirable please choose one of these for your answer: Android, iOS, or Veracrypt.
In general, if in spite of the way the post currently stands there is a beginning of a substantive question in it, please feel free to edit and improve it.