You explained just fine, but as i said the split files is a lot more common than the single big file. It lets studios put multiple versions of the same movie (theatrical cut, extended cut, director's cut...) all on the same disc without needing a lot more space. All the studio's need to do during authoring when doing it the split files way, is create a playlist per version, who then load the extra scenes in the proper order. This usually only adds a couple hunder MB to a few GB to the final disc size.
So with split files, assume the theatrical cut is 18GB in size, the extra footage for the extended cut is 2gb, and another 1gb for the directors cut. Now, 18+2+1GB = 21gb for 3 different versions of the same movie on 1 single disc (that could fit on a single layer disc).
If they were to do it the "1 big file per version" way, that would mean theatrical cut: 18gb, extended cut 20gb, directors cut 21gb for a total of 59GB, which is ABOVE what a standard BD can hold (max double layer BD is 45.5GB). Which would mean they either need to go into uhd triple layer discs, or split the 3 versions across 2 or 3 seperate discs, adding costs.
Which way is more logical profit wise?
The fact that you've never encountered this structure before is well, sheer dumb luck, "Seamless Transition" (what the split files way is called) has been around for years.