To add a custom field in Contacts in Odoo UI:

Using Odoo Studio (easiest):

1. Open any Contact → Click the Studio icon.
2. Drag a New Field onto the form.
3. Set label, technical name (x_...), and type.
4. Save and exit Studio and done!

You can also create a custom field in Contacts by enabling Developer Mode. These are the main steps for creating custom field contacts

1. Go to Settings > Activate Developer Mode.
2. Navigate to Contacts > Configuration > Contacts Fields (or go directly via developer tools).
3. Create a new field by setting its name, field type, and model.
4. Save your changes in your custom field will now be part of the Contact form.

To create a custom field in any form whether its contact or any other , you can use odoo studio app.
1- Install it from apps.
2- Open contact form.
3- In upper bar you will see a wrench icon, click on it
4- Contact Form editing view will be opened in Studio app
5- In left you will have editing panel from where you can drag and drop different custom fields in form and in same panel you have options(Properties), when you select any field on form, to change its type, name etc.
6- After adding your custom fields simply click close button in top right corner and exit.

New custom field will be added to form

Hope you understand it well!