Lookup and add description from another table

This seems like a basic ask and I’m sorry to bother people with something this basic but I can’t accomplish this with my limited experience with Airtable.

Vendor List - one particular field (TCODE) is a number (2 digit integer) that corresponds to a county in Calif.
Counties - 2 fields TCODE and TDESC which is string with the county name ie “Los Angeles”

Use Case
Import a spreadsheet of Vendors and their addresses which include the TCODE value
Somehow (linked tables?) somewhere (same table new field, new table?) add the county name from the Counties Table TDESC field.

My outcome so far…
I can link the tables with the TCODE field. That column now show the highlighted TCODE. It also creates new entries in Counties table (not good). I can add a lookup field in “Vendor List” and point to “TDESC” in Counties table but nothing is populated until I manually add the record from Counties to the linked field in Vendor list. My expectation is that there’s got to be a way to use existing data entered in Vendor List TCODE field to look up and return the TDESC string in the Counties table. Am I crazy?