Do you lend someone money or do you loan it?
You can do either, in fact. (The noun is always loan.)
The verb lend, in the sense of granting someone else temporary possession of something in the expectation of its eventual return, is an old one: Ælfric used it in his Grammar more than a thousand years ago.
Loan as a verb isn’t much more recent, going back at least as far as the early thirteenth century. In modern usage, however, the OED says it is ‘chiefly US’.
How we use the two verbs in this northern part of North America is, I think, idiomatic – with loan more frequent (but not universal) in a transactional setting.
Can you lend me ten bucks?
I will lend you the book, but please return it.
She lent me a hand when the going was tough.
The room doesn’t lend itself well to groups of more than ten.
Banks lend money at interest.
The finance company loans money at exorbitant rates.
The National Gallery loaned/lent the painting to the travelling exhibition.