I have searched, but have not yet found a solution:
How could I have alternating singer systems on the same page as in the attached image?
So that, for example, two or three voices of a choir start or vice versa and the other singers are not displayed with empty lines full of rests, but the brackets simply change alternately with increasing or decreasing “instrumentation”?
The distances between brackets should remain the same if possible despite changing instrumentation, without having to create new flows?
Thank you for your help!
Aha, OK, I have found it finally in a video with John:
https://www.youtube.com/live/voLK7fy_xLo?si=gmF8ZNkZG7eyqJ0J
It is possible using system breaks (create them in engrave mode at the position you want to change the visibility of staff):
In Write or Engrave Mode, double click on an existing “system break” and set
“manual staff visibility” in the window that opens. 
Make sure that system breaks are shown in
Dorico program- Main menu/View:
Signposts/system breaks
Great!
Thank you!
PS
If double-clicking on a system break e.g. in the first bar opens the text editing field instead of the staff visibility editing window, first go to Engrave Mode.
Correct the alignment or the frame of text near the system break in the page templates so that the system break can be clicked again and the staff visibility can be edited. Otherwise you could be unable to access the staff visibility by double-clicking on it.
In my case, on the first page, e.g. the opus number was on the left near the first bar of the score, where I wanted to make a staff visibility change in the first bar, so that the text editing window opened instead of the visibility window, although the staff break was clearly marked dark. 
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