The layer update (from Ikoria) only affects layer 1, giving it a couple sublayers. I don’t think that layers are really all that intuitive, at least not at first. Once you get a hold of it, I think this interaction still trips people up. The key to it is that all the relevant rules are not all in the same place, where they talk about layers.

In the majority of cases, information about what was on the battlefield first in very important when it comes to questions about layers since each layer/sublayer follows a timestamp order. That is not the case with the Urborg, Tomb of Yawgmoth and Blood Moon interaction.

This interaction breaks timestamp ordering due to a dependency (613.8). In other words, you will get the same result independent of the order that the two cards entered the battlefield. There are three requirements for a dependency, as defined in 613.8a. Let’s walk through each of them.

The first and hardest to understand criterion requires that two effects must apply in the same layer/sublayer. Since Blood Moon makes Urborg a Mountian and removes Urborg’s ability, it would seem like Blood Moon affects two layers, at first glance. Unfortunately, this is not the case. From conversations with other judges, the best way I’ve found to explain why comes from rule 305.7 (sited below). In short, a land that becomes a basic land type loses its text then and there (barring wording like “in addition”). While it might seem counter-intuitive, this would cause Urborg to lose its ability to give lands the Swamp subtype in layer 4. So, that’s the first requirement.

The second is much easier to grasp. Blood Moon is trying to strip Urborg of its ability. Meanwhile, Urborg is trying to giving lands the Swamp subtype. This forms a dependency in Blood Moon’s favor. Since applying Blood Moon’s effect removes the existence of Urborg’s, Blood Moon is applied first.

Lastly and most clearly, neither of these are CDAs, so we don’t need to worry about that.

There is a way to get around this interaction. If you can somehow give Urborg a land subtype “in addition to its other types” though an effect that isn’t a static effect from a land (like Urborg’s own ability), Urborg doesn’t lose its ability. This is a tricky interaction and an entirely different question, but there is a way around it.

At any rate, I hope this helps!!

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613.8. Within a layer or sublayer, determining which order effects are applied in is sometimes done using a dependency system. If a dependency exists, it will override the timestamp system.

613.8a An effect is said to “depend on” another if (a) it’s applied in the same layer (and, if applicable, sublayer) as the other effect; (b) applying the other would change the text or the existence of the first effect, what it applies to, or what it does to any of the things it applies to; and (c) neither effect is from a characteristic-defining ability or both effects are from characteristic-defining abilities. Otherwise, the effect is considered to be independent of the other effect.

305.7. If an effect sets a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text, its old land types, and any copy effects affecting that land, and it gains the appropriate mana ability for each new basic land type. Note that this doesn’t remove any abilities that were granted to the land by other effects. Setting a land’s subtype doesn’t add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.