Solved! Go to Solution.
As it relates to using Union's or Join's, Union adds rows, Joins add columns (in most cases).
So I think all it's missing is just another Union tool after that last Join tool in the workflow I shared last. Join adds in your new columns from the other input file and the Union brings back in all the other records where a match wasn't found.
Here's the result which I sorted on the "Security_ID" field to confirm that it's only showing up once and looking at the right side, you see the additional fields that were joined in from that additional input. All unjoined records from the left are retained
Lastly, being new to Alteryx, but looking like you're a proficient Excel user, I'd strongly encourage you to run through the interactive lesson linked below. Shows how certain/common features in Excel translates to Alteryx.
There's also a ton of other on demand videos you can look through. tip, use the "Labels" on the right side to filter down to a specific keyword (click the next to show additional).