There are two ways to choose different GoogleService-Info.plist. One is recommended in the official documentation as Run Time Configuration. The other is what I use to replace configuration before compile using Run Script.

Here is the official documentation: Configure multiple projects  |  Firebase

Run Time Configuration

For run time configuration, navigate to Configure multiple projects  |  Firebase for official documentation.

Compile Time Configuration

This is what I choose to do and how I do it. You can choose to use this method or not by your own choice.

First download all the GoogleService-Info.plist into your project. You can put it anywhere you want. Remember append something at the end of the file name. For example:

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-- Main Project
| -- Project Folder
| -- AppDelegate.swift
	| -- Firebase
		|-- GoogleService-Info-uat.plist
		|-- GoogleService-Info-release.plist

When adding files into the project, choose the correct target file is corresponding to. (You can also choose all targets if you want).

Then in the Target -> Build Phases, create a Run Script, drag it above the Copy Bundle Resources, and write the following script:

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PATH_TO_GOOGLE_PLISTS="${PROJECT_DIR}/<1>/Application/Firebase"
cp "$PATH_TO_GOOGLE_PLISTS/GoogleService-Info-<2>.plist" "${BUILT_PRODUCTS_DIR}/${PRODUCT_NAME}.app/GoogleService-Info.plist"

Replace: ::<1>:: with project folder name. ::<2>:: with the correct filename you appended.

The script will copy the plist file into the build binary root.

Why build binary root? You can of course copy to your project root before compile stage. However, it will then create a git difference. Copy to the binary file can direct avoid this problem.

By default, FirebaseApp.configure() will load the GoogleService-Info.plist file bundled with the application.

Therefore, this method works.

I use this method in Lowes Loyalty app when working for Kinetic.