In Darktable, a collection is a set of filters. The most common filter is “film roll” which by default is equivalent to an operating system’s file directory/folder. But a collection can be any of the other options or a combination of filters. And pictures can belong to multiple collections.
For me, tags are the most useful way of creating collections across multiple film rolls and date ranges are the second most useful. I can collect all my pictures tagged “New Mexico” on the light table if I select pictures by tag instead of by film roll.
If “New Mexico” is a useful collection, I can save it as a preset for easy access. If I am only interested in New Mexico pictures from 2018, I can narrow the search by date. With recent versions of Darktable I can even narrow further by lens or date modified or aperture , etc.
Tagging of course is a whole different can of worms. Some people want a grand hierarchy beforehand. I just tag as a go (or forget to) and live with a certain degree of chaos that comes from six years learning to use Darktable while using it to solve the tasks at hand.
Basically a Darktable collection is a set of search criteria. Tags are keywords and the other options are like “advanced search” options commonly found in search tools. The default collections by “film roll” is convenient for ordinary importing and editing, but falls apart for thematic collections.