Note, URLs are technically required, but I don't care. Just be consistent. Either include URLs or don't. But don't do both.

a DOI will appear like
10.1234/fake.2020.8.28
and should go between
the database name, and
the word "Accessed."

You could use Purdue, but Excelsior is a much better choice now. Purdue has some weird pop-up stuff.