Behavior on uploaded files

I am currently working on the security of uploaded files, which are stored in the upload folder. I have a few questions about that.

If I see it correctly, a timestamp will be added before the file name. For short file names (eg “cv.pdf”) I think the resulting name is too short. Would it be possible to increase the length of the prefix?

What happens if two users upload a file at the same time? Is there any exception handling for this? Maybe it makes sense to use the database ID?

With an empty index.php, .htaccess and corresponding permissions of the parent folder, access for long file names is relatively well secured. But if the files do not have to be accessible, the files could be stored in a non-accessible folder. Is it possible to change the path?

Best regards

André Kornetzky