Paypal transactions not going through with standard-integration

Im trying to implement purchasing with paypal
https://github.com/paypal-examples/docs-examples/tree/main/standard-integration

I set up a paypal business account and entered my client-id and secret into the env variable and into the Index.html (I assume its supposed the be the same client ID in both, correct?)

I followed the instructions, except I didn’t log in with my sandbox ID, instead I just sent money with CC

Create an application
Rename .env.example to .env and update CLIENT_ID and APP_SECRET
Replace test in public/index.html with your app's client-id
Run npm install
Run npm start
Open http://localhost:8888
Click "PayPal" and log in with one of your Sandbox test accounts

I have a couple issues:

  1. I followed the instructions npm install and npm start
    I tried sending the default amount to myself, but I dont see anything in the account, eventhough all of the transaction steps were completed in the paypal popup, I entered my CC info and etc. Am I missing something? Is the backend automatically started if I follow those instructions, or maybe it’s not starting the backend?

  2. I cant seem to change the payment value from the front end,
    I did a search and found value: "100" in paypal-api.js, and set it there to $1,
    I want to set the price using an external shopping cart, how can I set the value from Index.html?

  3. What does the sandbox account actually do? I don’t see any instructions on how to use it, I have a username and password for it, does it send money from myself to myself?

Thanks,