This is a noob question, but how do web applications (typically) ensure that they only show the users data once they are logged in?

For example, when logging in to an online bank, you see your own bank account data. What is the application doing with the user/identity it has recieved from the browser to query and present the correct data to the website – which would typically be stored in a database, but could also be on other storage such as S3?

I assume the application uses an index on the database which is the user identifier, so you can query for each users data specifically and return to the web app?

Is there anything else going on here?

I tried googling, looking through stackoverflow and asked chatgpt, but didn’t get a satisfactory answer.