Good Morning, @Demitry
I am so glad I thought to revisit this thread.
I never did install the XS module for this contribution. The Categories button has always displayed on cellphones, despite never having installed the XS folder.
And when clicked, the Categories button always displayed the parent categories with arrows to indicate there are deeper categories within, so...I just assumed everything was okay. I am not a cellphone person. I hate cellphones, so...I thought what I was seeing was sufficient and that was the end.
If I clicked on an arrow next to a category (indicating there are sub-categories) - nothing would load on a cellphone. Couldn't see the subcats at all. It's been like that for a year and I had no idea until today.
So, I installed the XS folder and...what a difference, and it's a huge difference. Everything is appearing beautifully, complete with a back button to go back to the parent category. God, I wish I had realized how this works a long time ago.
The instructions were pretty vague and just said, you can install one or the other or both. And that's about it. So, I installed the BS folder only. I didn't understand the significance of the XS folder at all.
Big mistake in a world full buyers using cellphones to shop. Glad you pointed me in the right direction - Thanks, Demitry!