vlookup type relationship

I have two source tables that share Interal Order to tie them together. The first table (called InternalOrder) has all 500K in our ecosystem, whereas the second table (IO Used Table...) has a subset of the total Internal Orders (maybe 100k). I assume because it isn't a 1:1 match because one table has less than the other, I can't use the "Related" function. I want to bring a specific field in the table that has only 100K into the all-encompassing table that has all 500K but can't figure out how.