@Pat_Maddox has given an excellent overview already, let me add some views from our end.
Indeed, calendar dates and events can be assigned/linked to a whole note but not individual paragraphs within a note, while reminders are the way to assign (due)dates to individual paragaphs in a note. We on purposely keep this separation to avoid confusing about what to use when, and also to avoid an overwhelming amount of options and associated UI (the two go hand in hand obviously).
Currently there is not yet a way to get an overview of due reminders or unchecked checklist items, both are on our Wishlist to be added down the line.
And alternative is the route that Pat suggested, to use so-called due tags, tags that have an associated date to them. For these you can make smart overviews similar to the today overview.