To transfer data from a lead custom field to an opportunity field upon lead conversion, an administrator needs to create a custom field on the opportunity object that matches the data type and length of the lead custom field, and then map the two fields using the lead field mapping tool under setup. This will ensure that the value of the product category field on the lead is copied to the corresponding field on the opportunity when the lead is converted. References: https://help.salesforce.com/s/articleView?id=sf.leads_customize_map.htm&type=5

Section: Volume J

A formula field is a type of field that calculates a value based on an expression or formula that references other fields or constants. For example, a formula field can display today's date by using the TODAY() function. In this case, the administrator can create a formula field on the account object that updates the audited date field with today's date when the status of the account is changed to 'Audited'. References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_formulas.htm&type=5