Explanation
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