Each land is a Swamp in addition to its other land types.
"Yawgmoth's corpse is a wound in the universe. His foul blood seeps out, infecting the land with his final curse."
—Lord Windgrace
(19/03/2021)
If an effect such as that of Magus of the Moon causes Urborg to lose its abilities by setting it to a basic land type not in addition to its other types, it won't turn lands into Swamps, no matter in what order those effects started to apply.
(19/03/2021)
Urborg, Tomb of Yawgmoth isn't a Swamp while it's not on the battlefield.
(19/03/2021)
Land cards not on the battlefield aren't Swamps while Urborg is on the battlefield.
(19/03/2021)
Urborg's ability causes each land on the battlefield to have the land type Swamp. Any land that's a Swamp has the ability ": Add ." Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they're legendary, basic, or snow.
