How to choose the loop all at once in one click?
Usually I know how, but in this case, it don't. work. https://drive.google.com/file/d/1wJGAwchcNQL4-UfBtCWBNl5mCaAdeTOc/view?usp=sharing
I think 
spikeyxxx is correct,
If you look closely you can see there are edges visible that are grey.
When a face, or face loop is selected, all edges of the selected faces should be orange (or white for the active face).
Look, I deleted 60 extra vertices with Merge by distance, and I still can't properly select the hole loop in one shot. :-(
my god. How can I designed such a bad topology??? thx, nice trick to hide some parts.
