Just like any other file Moodle stores, the backup files are stored in moodledata/filedir/{dir1}/{dir2}/ where {dir1} and {dir2} are the first two and the next two characters of the hash of the file. A hash is a seemly random sequence of letters and numbers so you can't tell just from the file name which is the file you're after. Your hashed file name might start with 83b99e... so in this case {dir1} would be 83 and {dir2} would be b9.
However, there are two things you can look for (without digging in the database) that will help you guess which is the file you're after: the time stamp and the file size.
Moodle backup files are usually large, anywhere from a few hundred kB to well over 10MB. If you know when the backup file was created you can use that to narrow down your search too. If your file is the only file in its directory then the directory will have the same time stamp too so that might save you having to look through all the directories.
Once you think you have downloaded the correct file you can check it by opening it in a ZIP extraction program.
There is a lot of inspired guesswork in this method, so it might take a while for you to find the right file, however, it is not impossible and depending on your circumstances can often be done quickly.