They generally record 5.1 channels onto DVDs. Your receiver decodes the data to get these 5.1 channels and it plays them for you. That's it.

Look on the back of the DVD box to see how it is recorded. Movies were generally always mono up until the early 80s and then stereo until sometime in the 90s when 5.1 came around. So many older movies are still in their original soundtrack on DVDs, which they should be.