Each land is a Swamp in addition to its other land types.
Does Urborg, Tomb of Yawgmoth work on lands in libraries?
I.e., does it allow me to fetch any land using a Swamp-fetcher?
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No. Urborg, Tomb of Yawgmoth only affects lands on the battlefield, not in any other zone.
Rule 109.2 says
If a spell or ability uses a description of an object that includes a card type or subtype, but doesn't include the word "card," "spell," "source," or "scheme," it means a permanent of that card type or subtype on the battlefield.
Urborg affects objects with the type "Land". Since it doesn't use any of the mentioned words, that means that it affects permanents of type "Land" on the battlefield.