A couple of nooby questions about built-in and custom forms

[eddz99]

Questions Joomla 3,
1. How would I go about editing the registration form that will make it have specific custom fields submit to another custom table?

2. How would I add a custom field in the core registration form wherein the field will check if a value exists in the users table?

3. And how will I also implement it in the edit profile function?

Also,
4. How can I create a custom form that will write to a table that I manually created?

5. Also how can I display the table's data on the front end as well as in the admin panel?

Thank you!

[Per Yngve Berg]

Write a User Profile Plugin

https://docs.joomla.org/Plugin#Using_Plugins