How to collect statistics on a one-page site?

Question

HTML

Finally got around to Piwik. I really liked how he works on the project.
I decided to tie it to the previously made one-pagers. And then the question arose - how to understand which sections users are watching?
The structure is approximately the following...
When entering the site, a modal window pops up - select a branch. After selection, a page with data from the database for this particular city is loaded. The URL remains the same as mysite.ru. Accordingly, I can not understand which branches are watching and which are not. When navigating through sections (scroll), the URL does not change either. Moreover, only the text of this modal window is indexed in Yandex...
Now I plan to slightly update the structure of these sites - I would like to take into account these nuances.
What can you advise?

Answer 1

I can’t tell you specifically about Piwik, but I think it should be like this: with Google Analytics, I would simply add an event to be sent on click to the selected branch with the name of this branch sent

Answer 2

to previously made one-pagers.

One contradicts the other. A one-page site contains 1 page, from the word ONE.
Add a custom field with the types of your sites and steer in relation to these slices, otherwise nothing.