Save the user session in flask and create a function that requires the user to be logged in. Then reference said func.
You will need to create a custom route for the user to edit their information based on how that data is stored. (I.e ‘www.example.com/<user_id>) will query all of the customers info from your database. Then update the data.
On signup you can go for a more static approach for simplicity. But on button type “submit” you will need to create a check by selecting usernames from database if a result is returned show a flash alert that account exists. A cleaner approach would be to us JavaScript.
But as stated, you are a long way from this all coming together and working adequately. Best of luck!