Change contents of cells based on a drop down

So, I'm trying to generate a spreadsheet to make the forms we use in the office to issue keys.

When people are given an apartment, we have a sheet with all the key numbers on it, and then they sign for it.

At the moment, we've got 22 different PDFs and Word documents, and it doesn't work.

So, what I want to do is this.

I've built the template in Excel and created a Reference sheet which will have all the numbers in it.

What I am trying to do is set it up so that when you use the Apartment number from a dropdown, it populates with all the key numbers.

This sounds like it should be easy to do, but my Excel brain is broken.

Advice would be much appreciated!