It doesn't matter.

URI is basically a format and nothing else. (foo://bar123 would be a URI but not a URL because nothing defines what foo: is.)

URLs and URNs are thingies using the URI format; https://news.ycombinator.com is a URL (in addition to being a URI) because there's an RFC that specifies that https: means and how to go out and fetch them.

urn:isbn:0451450523 (example cribbed from Wikipedia) is an URN (in addition to being an URI) that uniquely identifies a book, but doesn't tell you how to go find that book.

Mostly, the difference is pedantic, given that URNs never took off.