How to set up multiple schemes & configurations in Xcode for your React Native iOS app
Alek Hurst

Thanks for the article, it was very helpful.
Although I have encountered some problems with sync-build-configs.rb. Above solution is only adding missing configuration to project, but configuration are also found in targets (not visible in Xcode!!!). Not sure if this is a new feature from Apple.
Here’s my complete (slightly altered) solution:
What I generally did was:

  1. Added function to XCodeProject, ensure_build_config_in_targets
  2. Modified fixing projects to:
    @mappings.each do |kind,names|
     names.each do |name|
     # first fix targets and later project
     project.ensure_build_config_in_targets(name, kind)
     project.ensure_build_config(name, kind)

Note that targets (of a single project) have to be fixed before fixing the project itself. This solved error when executing script.

After this steps I was able to add AdHoc config to all my libraries and build AdHoc successfully.