Does Magus of the Moon affect lands that have been given basic land types by another effect?

Magus of the Moon has an ability that reads: nonbasic lands are Mountains. This is a continuous effect that operates in the type-changing layer (Layer 4) and the ability-adding/removing layer (Layer 6) of the layer system defined in CR 613.

If another effect has previously granted basic land types to a nonbasic land — for example, Urborg, Tomb of Yawgmoth making all lands also Swamps — Magus of the Moon's effect still applies. Because both effects operate in Layer 4 (subtype changes), they are applied in timestamp order per CR 613.7. Magus of the Moon's effect, if it has the later timestamp, overwrites the previously granted types, making the land just a Mountain. If it has an earlier timestamp, the other effect could add back a subtype, but Magus's effect still removes the previous types first.

Importantly, in Layer 6, Magus of the Moon strips all non-Mountain abilities from affected nonbasic lands. So even if a land ends up with multiple subtypes due to timestamp interactions, it loses its activated mana abilities that came from rules text (CR 613.1f, 702.2).

Concrete example: You control Urborg, Tomb of Yawgmoth (entered first) and your opponent controls Magus of the Moon (entered second). Your nonbasic lands become Mountains due to Magus's later timestamp. Urborg itself is a nonbasic land, so it becomes a Mountain, loses its Swamp-granting ability, and produces only red mana.

The key takeaway: Magus of the Moon affects ALL nonbasic lands regardless of what other effects have been applied to them, with final results determined by the layer and timestamp system (CR 613.7).