To get more data from related fields in the REST API, you can use the fields parameter in the API call. This parameter allows you to specify which fields you want to include in the response. For example, if you want to get the username field from the res.users model, you can use the following API call:

GET /api/users?fields=id,name,username

This will return a response with the id, name, and username fields for each user in the database. You can specify any fields you want in the fields parameter, separated by commas.

Keep in mind that not all fields are available in the REST API by default, and you may need to enable them in the ir.model.fields model before you can use them in the fields parameter. You can do this by setting the readonly attribute to False for the desired fields.

I hope this helps! Let me know if you have any other questions.

Take a look to fields_get() method here: https://www.odoo.com/documentation/15.0/es/developer/api/external_api.html#list-record-fields