Here's an example of how you can create a dynamic formula in Odoo using the "onchange" method to show a second field based on the selection of "Phone Brands" attribute:
 

This code creates a "Customer" model that has two fields: "phone_brand" and "iphone_model". The "phone_brand" field is a selection field with three options: "Apple", "Samsung", and "Google". The "iphone_model" field is a selection field with three options: "iPhone 11", "iPhone 12", and "iPhone 13".

The onchange decorator is used to specify that the _onchange_phone_brand method should be called when the "phone_brand" field is changed. The method checks the value of the "phone_brand" field. If it is "Apple", it sets the "iphone_model" field to False, which will make it visible. If it's not "Apple" the "iphone_model" field is set to false and the field will be invisible.

You can adjust the code to fit your specific requirements, but this should give you an idea of how to create a dynamic formula using the "onchange" method in Odoo.

You can create correlated drop-down lists for customers selection in your e-commerce store by using the domain attribute of the fields.Many2one field.

Here is an example of how you might implement this in your e-commerce store:

First, create a new model for the phone brands, for example ecommerce.phone.brand with a field name as string.

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class EcommercePhoneBrand(models.Model):

    _name = 'ecommerce.phone.brand'

    name = fields.Char()

Create a new model for the phone models, for example ecommerce.phone.model with fields name as string and brand_id as Many2one to the phone brands model.

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class EcommercePhoneModel(models.Model):

    _name = 'ecommerce.phone.model'

    name = fields.Char()

    brand_id = fields.Many2one('ecommerce.phone.brand')

In the customer model, create a field phone_brand_id as Many2one to the phone brands model and a field phone_model_id as Many2one to the phone models model.

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class Customer(models.Model):

    _name = 'res.partner'

    _inherit = 'res.partner'

    phone_brand_id = fields.Many2one('ecommerce.phone.brand')

    phone_model_id = fields.Many2one('ecommerce.phone.model')

Then you can add the domain attribute to the phone_model_id field, which will limit the options in the drop-down list based on the value of the phone_brand_id field.

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phone_model_id = fields.Many2one('ecommerce.phone.model', domain="[('brand_id', '=', phone_brand_id)]")

Then you can add the fields to your customer form view, and when you select a brand from the phone_brand_id field, the phone_model_id will automatically update to show only the models of the selected brand.

It's important to note that this is just an example, you will have to adapt it to your specific use case, and also, you will need to create the phone brand and model records in the backend before you can use them.

It's also important to note that you will need to handle the case when the phone_brand_id is not selected yet, otherwise, the phone_model_id will not work as expected.