Don't understand the URL creatred by a checkbox in HTML

So I am a little newbie doing web scrapping and I am facing a challenge I do not find any solution for.

The idea is to know the "/xxx" extension that a link provides to a URL. For the first time, it was enough to look at the attribute 'value' of the object of the HTML source to know what was going to be added to the URL.

But this website is a little more tricky:

On this website you can select one or more drugs, and gives you the kind of interaction between the rest of them. Its URL is https://www.hiv-druginteractions.org/view_all_interactions/new.

When I click on the one called Atazanavir/cobicistat, I get this URL: https://www.hiv-druginteractions.org/view_all_interactions/401621

These numbers added (401621 and 401624) do not appear in the attribute value of the drug when inspecting the HTML code. Can someone explain to me how are those numbers generated?

Also, can you provide me a link to sources that talk about this topic?

I apologize if this is a very simple question but I do not know how to solve it.