What is kind of neat is every windows application/dll is a valid DOS application. The first part of all of them is a valid MZ DOS 16 EXEcutable. Windows just treats it as a skippable header and reads the real header that is about 100 bytes in and then decides which subsystem to fire up (win3x, win32, OS/2, etc). But if you take a exe compiled today with current tools and put it on a DOS 3.3 box it would run the exe and print out it can not run (the exe has that in there).

Also from that era not all DOS applications were exclusively 16 bit. Many were hybrid. Just to have better control over the memory space instead of using segmentation was usually worth the speed boost (as well as the bigger registers). Windows from that era usually had extra 'pid' file where you could basically tag the executable as 'hey you are about to run a 32 bit app get out of the way windows oh and support dpmi while you are at it'.