A shell script to selectively copy your GoogleService-Info.plist into your app bundle based on the current build configuration.

alternative-copy-google-service-info-plist.sh

if [ "${CONFIGURATION}" == "Debug-Production" ] || [ "${CONFIGURATION}" == "Release-Production" ]; 
then
    cp "${PROJECT_DIR}/Runner/Firebase/Production/GoogleService-Info.plist" "${PROJECT_DIR}/Runner/GoogleService-Info.plist"
    echo "Production plist copied"

elif [ "${CONFIGURATION}" == "Debug-Uat" ] || [ "${CONFIGURATION}" == "Release-Uat" ]; 
then
    cp "${PROJECT_DIR}/Runner/Firebase/Uat/GoogleService-Info.plist" "${PROJECT_DIR}/Runner/GoogleService-Info.plist"
    echo "Uat plist copied"

elif [ "${CONFIGURATION}" == "Debug-Develop" ] || [ "${CONFIGURATION}" == "Release-Develop" ]; 
then
    cp "${PROJECT_DIR}/Runner/Firebase/Develop/GoogleService-Info.plist" "${PROJECT_DIR}/Runner/GoogleService-Info.plist"
    echo "Development plist copied"

elif [ "${CONFIGURATION}" == "Debug-Sit" ] || [ "${CONFIGURATION}" == "Release-Sit" ]; 
then
    cp "${PROJECT_DIR}/Runner/Firebase/Sit/GoogleService-Info.plist" "${PROJECT_DIR}/Runner/GoogleService-Info.plist"
    echo "Mock plist copied"

fi

copy_appropriate_google-service-info-plist.sh

# Name of the resource we're selectively copying
GOOGLESERVICE_INFO_PLIST=GoogleService-Info.plist

# Get references to dev, sit, uat, prod versions of the GoogleService-Info.plist
# NOTE: These should only live on the file system and should NOT be part of the target (since we'll be adding them to the target manually)
GOOGLESERVICE_INFO_DEV=${PROJECT_DIR}/${TARGET_NAME}/Firebase/Develop/${GOOGLESERVICE_INFO_PLIST}
GOOGLESERVICE_INFO_UAT=${PROJECT_DIR}/${TARGET_NAME}/Firebase/Uat/${GOOGLESERVICE_INFO_PLIST}
GOOGLESERVICE_INFO_SIT=${PROJECT_DIR}/${TARGET_NAME}/Firebase/Sit/${GOOGLESERVICE_INFO_PLIST}
GOOGLESERVICE_INFO_PROD=${PROJECT_DIR}/${TARGET_NAME}/Firebase/Production/${GOOGLESERVICE_INFO_PLIST}

# Make sure the dev version of GoogleService-Info.plist exists
echo "Looking for ${GOOGLESERVICE_INFO_PLIST} in ${GOOGLESERVICE_INFO_DEV}"
if [ ! -f $GOOGLESERVICE_INFO_DEV ]
then
    echo "No Development GoogleService-Info.plist found. Please ensure it's in the proper directory."
    exit 1
fi

# Make sure the sit version of GoogleService-Info.plist exists
echo "Looking for ${GOOGLESERVICE_INFO_PLIST} in ${GOOGLESERVICE_INFO_SIT}"
if [ ! -f $GOOGLESERVICE_INFO_SIT ]
then
    echo "No SIT GoogleService-Info.plist found. Please ensure it's in the proper directory."
    exit 1
fi

# Make sure the uat version of GoogleService-Info.plist exists
echo "Looking for ${GOOGLESERVICE_INFO_PLIST} in ${GOOGLESERVICE_INFO_UAT}"
if [ ! -f $GOOGLESERVICE_INFO_UAT ]
then
    echo "No UAT GoogleService-Info.plist found. Please ensure it's in the proper directory."
    exit 1
fi

# Make sure the prod version of GoogleService-Info.plist exists
echo "Looking for ${GOOGLESERVICE_INFO_PLIST} in ${GOOGLESERVICE_INFO_PROD}"
if [ ! -f $GOOGLESERVICE_INFO_PROD ]
then
    echo "No Production GoogleService-Info.plist found. Please ensure it's in the proper directory."
    exit 1
fi

# Get a reference to the destination location for the GoogleService-Info.plist
PLIST_DESTINATION=${BUILT_PRODUCTS_DIR}/${PRODUCT_NAME}.app
echo "Will copy ${GOOGLESERVICE_INFO_PLIST} to final destination: ${PLIST_DESTINATION}"

# Copy over the prod GoogleService-Info.plist for Release builds
if [ "${CONFIGURATION}" == "Debug-Production" ] || [ "${CONFIGURATION}" == "Release-Production" ]; 
then
    echo "Using ${GOOGLESERVICE_INFO_PROD}"
    cp "${GOOGLESERVICE_INFO_PROD}" "${PLIST_DESTINATION}"

elif [ "${CONFIGURATION}" == "Debug-Uat" ] || [ "${CONFIGURATION}" == "Release-Uat" ]; 
then
    echo "Using ${GOOGLESERVICE_INFO_UAT}"
    cp "${GOOGLESERVICE_INFO_UAT}" "${PLIST_DESTINATION}"

elif [ "${CONFIGURATION}" == "Debug-Develop" ] || [ "${CONFIGURATION}" == "Release-Develop" ]; 
then
    echo "Using ${GOOGLESERVICE_INFO_DEV}"
    cp "${GOOGLESERVICE_INFO_DEV}" "${PLIST_DESTINATION}"

elif [ "${CONFIGURATION}" == "Debug-Sit" ] || [ "${CONFIGURATION}" == "Release-Sit" ]; 
then
    echo "Using ${GOOGLESERVICE_INFO_UAT}"
    cp "${GOOGLESERVICE_INFO_UAT}" "${PLIST_DESTINATION}"

fi

Reference: https://medium.com/atlas/using-multiple-firebase-environments-in-ios-12b204cfa6c0