How to add a custom field to the frontend registration form?

How can Customize Registration Form to add Fields in Magento2

It takes a lot of work.
See the following articled for a start:

• How is the customer registration form («Create New Customer Account») implemented?
• How is the «Sign Up for Newsletter» checkbox implemented on the customer registration form («Create New Customer Account»)?
• customer-registration-tagged articles

You can add your custom field to the form_additional_info container:

How to add a custom field to the frontend registration form (a ready example)

How do built-in Magento modules add a custom block to the frontend customer registration form?