The master is a four track tape and was then tinkered from for making the 70mm Dolby, 35mm Dolby and mono. It was reutilized in 1997 when the SE track was being made and then likely digitized as all the other mixes are supposedly on file.
The musical score is also on four track IIRC.
If I’m mistaken on anything feel free to correct me.
The 70mm 4 track and the 35mm matrixed stereo are essentially the same mix. The only difference is the 70mm also had an LFE track (or two LFE tracks - the info is unclear, but it is clear that other than LFE, there was L, R, C, and surround and those were identical to the matrixed tracks found in the 35mm). The Mono mix was done completely separate and featured more detail in most places, but lacked some of the effects applied for the stereo mix, such as the echo when Luke and Leia are trapped in the casm and the radio distortions during the Death Star battle. Now, one source I found indicated that the LFE is actually included in the matrixed stereo mix. Basically the surround and LFE are on the same channel, with everything below the range of 80-120 Htz being the LFE and everything above being the surround. But I have not been able to confirm this.
The great advantage of Dolby Stereo was that it worked for theaters with mono, stereo, and surround system, all from one master print. Where I live, only 2 of the 16 screens had stereo growing up. Since then they have built new facilities that all have surround and have kept up with the latest digital formats are are all DLP today. But that change was in the early 90’s, so most of the films I saw were in mono even though the film itself was stereo. But all those movies I saw in mono, if you run them through a Dolby Pro Logic system will have surround, either 4.0 or 5.0 depending on when they were released. That is why there really isn’t a separate mono mix for Empire or Jedi. No need. When Star Wars came out in 1977, some theaters didn’t have equipment that was compatible so Ben Burt made a special mono mix.