You have a total of 96 LEDs
They take up addresses 0 through 95 So yes you are correct with 0 being a value.
Segments are not really groups they are sections of that full 96 LEDs that still allow them to be individually controlled. It just splits them so that whatever effect does not continue along that full amount of 96, but rather lets sections act in sort of a parallel, but also letting you apply different effects to each segment if you so choose.
It does not matter how many data GPIO ports you use on the controller, WLED addresses all of the LEDs in sequential order. The software calculates what GPIO to send what data to.
Groups in WLED are when you want to group a number of individual LEDs so they act as 1 LED. If you set your grouping to say 3; you would then have 3 LEDs that act as 1.
You are creating segments based on all of your LED numbers.
Segment 0 is 0 to 16 because WLED is not including that last value. It is labeled as the ‘stop LED’ Meaning that it has stopped there and is not included in the segment.
Segment 1 now needs to start at LED 16 because you already have LEDs 0 through 15 in segment 0
Segment 2 starts at LED 32 because Segment 1 used 16 through 31 and segment 0 used LEDs 0 through 15.
and so on…
If you put all the segments as 0 to 16 all of them would be trying to control LEDs 0 through 15 and would look like a mess of overlapped data.