Read value of key from one json file to another json file

I am doing Cypress automation with JavaScript. In this, I want to read value of one JSON file to another JSON file. And based on that want to retrieve other values.

Below is my credentials.json file:

{
    "url": "site.url",
    
    "sausername": "sausername",
    "sapassword": "sapassword",
    
    "viewonlyusername": "vouusername",
    "viewonlypassword": "voupassword",
}

In the above file, I pass the URL of different environments where I want to run my scripts. ‘sausername’, ‘sapassword’, ‘viewonlyusername’ and ‘viewonlypassword’ are credentials of different users.

Below is the SuperAdminTestData.json file, which includes required test data for super admin script.

{
    "accountName": "account name",
    "PartnerAdminAccount": "Automation Partner",
    "WholesalePartnerAccount": "Automation Wholesale Partner",
    "mitelAccountName": "Platform name",
    "siteName": "site name"
}

Now I am using this data in SuperAdminSpec.js file as below:
describe(“Super Admin – User Roles and Permissions”, function () {

let superAdminTestData, credentials
before(function () {
    cy.fixture('credentials').then(function (testdata) {
        credentials = testdata
    })

    //Get test data from 'superAdminTestData' file
    cy.fixture('superAdminTestData').then((testData) => {
        superAdminTestData = testData
    })
})

it("Login with valid credentials", function () {
    loginfunction(credentials.url, credentials.sausername, credentials.sapassword) //data from json file
})

it('Navigate to Partner Admin account', function () {
    //Click on all customer dropdown
    clickAllcustomers()
    //Navigate to Partner Admin Account
    clickCustomer(superAdminTestData.PartnerAdminAccount) //data from json file
})

})

In this structure, when I want to run my scripts on a stage environment, I have to go and manually change all data in both JSON files (Actually I have more than two JSON files which require changes).

Requirement: I want to add an extra key and value "environment": "stage". So when I want to run my data on stage I just have to change value of that key only from test to stage and vice versa. If I pass stage, it will take value for stage.

Question: Can I read value of one JSON file into another? If yes, how?

Please note that I am a beginner in Cypress as well as in automation.

I have not tried anything. But yes go through some solutions from Google and stack overflow as well:

• Key from one Json file to value of another Json file using node/Javascript
• Jq – add a key/value to one JSON file if such key/value exists in another JSON file

but I can’t understand it. Please help me. Thanks!