Explanation
An external ID is a custom field that has the external ID attribute enabled, which means it can be used as an identifier when importing and updating records from an external system. It allows administrators to match records based on a unique ID value from another system instead of using Salesforce record IDs, which may not be available or consistent across systems. An auto-number field is a custom field that automatically assigns a unique numeric value to each record, but it cannot be used as an identifier when importing and updating records from an external system because it is generated by Salesforce and may not match with the external system's IDs. A rich text field is a custom field that allows users to enter formatted text, images, and links, but it cannot be used as an identifier when importing and updating records from an external system because it is not unique or consistent across systems. A record ID is an internal ID assigned by Salesforce to each record, but it cannot be used as an identifier when importing and updating records from an external system because it may not be available or consistent across systems. References:
https://help.salesforce.com/s/articleView?id=sf.custom_field_attributes.htm&type=5

Explanation
To routenew leads to the correct sales person based on the lead address, the administrator should use lead assignment rules that specify criteria based on lead fields such as City, State/Province, or Country, and assign leads that match those criteria to queues or users. Lead assignment rules can be triggered automatically when leads are created or manually by users. Creating a formula field, assigning with an escalation rule, or configuring a validation rule will not route leads to sales people.
References:https://help.salesforce.com/s/articleView?id=sf.leads_assignment_rules.htm&type=5