County Field on account Salesforce

Create a process that will populate the [login to view URL] field with the proper county based on account current zip code on in the mailing address field using the custom object County Zip. I have already created a county field on the account, I have created a new custom object County_Zip, which includes [login to view URL], Created By, Last Modified By, Owner, State_Name, State_ID, Zipcode, County_Name, Population, Time_Zone. I have uploaded the 2018 County list provided by the USPS to create a separate record for each county. I now need a process created that will look up from the Zip/postal code on the mailing address of the account record and populate the corresponding County_Name. I want a one time run that would populate all USA accounts by looking up from their mailing address zip/postal code (first five digits if they have the + 4 code). Then I would like a process builder or trigger that will fire anytime the mailing zip code is changed. See this for understanding [login to view URL]