Hello
"automatically configures your Xcode project" - that's not very developer-friendly. I suppose they have some sort of "DIY instructions" elsewhere to explain what needs to be done ?
*edit* found it, it's just a matter of switching the "platform" tab in their documentation from "Unity" to "iOS". Because that's pretty much the point here.
https://firebase.google.com/docs/crashlytics/get-started?platform=ios
So, what you need to do is to build the DEBUG configuration of the Xcode project. This will build the code without optimizations and create a .dSYM folder in the .ipa archive containing the same debug info that Xcode would generate.
After that, in step 4.c, they require to add a custom build step where a script they provide called "run" is run. This is most probably a POSIX/macOS shell script that won't run on Windows. What you need to do then is to read it, understand what it does, and convert it to a format that's executable on Windows, either by reimplementing its functionality in the Microsoft scripting language (Batch or PowerShell), or by creating your own implementation in whatever language of your choice, so that it can be run in the pre-packaging phase by the builder as a pre-packaging script.
Let me know if you need more help on this step.