I know that a creature is a permanent in play, not a creature card, and I expect that the same applies to lands. Urborg, Tomb of Yawgmoth says, "Each land is a Swamp in addition to its other land types." That doesn't apply to land cards in my library, right? Only lands in play?
Correct. In general, if "a spell or ability uses a description of an object that includes a card type or subtype", including creature, land, Goblin, Aura, etc., "but doesn't include the word 'card,' 'spell,' 'source,' or 'scheme,' it means a permanent of that card type or subtype on the battlefield" (C.R. 109.2, 205.2, 205.3). Thus, Urborg's ability applies only to lands on the battlefield, not land cards anywhere else.
One exception to C.R. 109.2 is protection abilities (C.R. 702.16a). Thus, for example, because Horizon Drake has "protection from lands", damage that land sources would deal to Horizon Drake is prevented and abilities from land sources can't target Horizon Drake, even if those sources are not on the battlefield (C.R. 702.16b, 702.16e). See also this thread.
One exception to C.R. 109.2 is protection abilities (C.R. 702.16a). Thus, for example, because Horizon Drake has "protection from lands", damage that land sources would deal to Horizon Drake is prevented and abilities from land sources can't target Horizon Drake, even if those sources are not on the battlefield (C.R. 702.16b, 702.16e). See also this thread.