What is the record to find the possible values for a list?

Should be the record that is used for that list. I.e if you have a list/record field where the source is “vendors”, you can search/query the vendor record.

There's another way to find the values if you want to do so outside a search. If you know the list appears on a specific record type:

let dummy = record.create(); <whatever record type you need, isDynamic:true
let myField = dummy.getField(); < field that you want the list of values from 
let theOptions = myField.getSelectOptions(); < you now have an array of objects with value and text key value pairs