Relations used to always show the first column in the related sheet, no matter what. At some point that changed to what seems like a random column that’s displayed. I don’t think any of us have ever pinpointed which column it chooses now, so it’s seemingly random.
As for your next question, normally I like to assign unique values to a user, and use that unique value throughout the app. You can create a value by adding a Row ID Column - Glide Library, which would automatically populate your members sheet with unique ID’s for each member. There is also a Unique identifier - Glide Library special value column that you can use when adding rows through the add function or through a form. I tend to use unique values like those mentioned because they don’t rely on a name or email which could potentially change down the road.
The hard part though is that if you already have sheets with data, it’s hard to properly create those links on the existing date. In my app, which has a list of students, I used to link related data between sheet based on student name only, but I always had concerns if a name ever had to be corrected later. Eventually I ended up adding a Row ID column to my students sheet. Since all of my data resides in google sheet, I then had to go through each of my additional sheets, create an arryformula with a vlookup formula to tie the name to the student sheet, then the vlookup would return the row id for each matching student. Finally I copied that rowid from the vlookup into its own column. Once I had that data, I was able to redo my existing relations to use the row ids from the student sheet to the other related sheets. Now, whenever I add data to the other sheet, I end up passing along the students row id via the Column values - Glide Library. That’s the hard way to deal with any existing data, but most future proof in the long run and in my opinion, it’s the best as long as you don’t have a need to use Row Owners - Glide Library, which requires an email address to work properly and secure data only for the intended user.
If you want to stick with your idea of simply joining two columns together, that very easy to do by creating a Template Column - Glide Library to join two columns together. With that, you can then use the template columns as your key in the relation column. Just simply join the template column in one sheet to a template column in another sheet.