How to add fields and change required fields in moodle signup form?

I have a moodle project as a developer and i want to change the sign up form to make it simpler.Bascially, i want to remove the username field and replace it with a full name field and add a phone number field that is not mandatory (required) and remove all the other unnecessary fields or at least set them as not required

I have hid some of the unnecessary features using CSS but i want something more functional, and want to know what files i need to change to make the changes in the backend so that it works as expected when a user signs up.
Any help/advice with this would be greatly appreciated!