Regarding your point in 2: do you store your docs in a directory called Documents or Document?
When you name the resource you are naming a directory/table. You pick a file/row in it by either using an ID or query parameters. That's how you reduce the many to the one.
Just like how you say "one of the students" (singular lookup, /students/42) or "students who study CS" (plural query, /students?studies=cs). Not "one of the student" (singular, looks ok: /student/42) or "student studying CS" (plural, but reads as singular: /student?studies=cs).