The Layer System
When several continuous effects apply to the same card, it is NOT the order of casting that decides, but the layer system (rule 613): a card's values are rebuilt from scratch, layer by layer. Within a layer, timestamp order applies — unless one effect depends on another (dependency), in which case the independent one is applied first.
Why "iceberg"? Because in the game you only ever see the final result — the six layers below that produce it stay under the surface.
The 7 Layers
Layer 7 in Detail
Example 1: Why order of casting doesn't matter (Layer 7)
A Grizzly Bears (2/2) gets enchanted: first "becomes 0/4", then Giant Growth (+3/+3), finally a switch effect. No matter what order the three were cast in — the result is always the same, because the sublayers fix the order.
Base value
The printed value of the Grizzly Bears.
7b: "Becomes 0/4"
Setting effects come first — the Bears are now 0/4, the printed value is overridden.
7c: Giant Growth +3/+3
Adding/subtracting comes AFTER setting — even if the Giant Growth was cast first! 0+3 / 4+3.
7d: Switch
Switch effects always come last: power and toughness are exchanged.
Grizzly Bears
2/2
0/4
3/7
7/3
Example 2: Blood Moon vs. Urborg (Dependency)
Blood Moon ("Nonbasic lands are Mountains") and Urborg, Tomb of Yawgmoth ("Each land is a Swamp in addition to its other land types") are on the battlefield at the same time. Both work in layer 4 — but the result does NOT depend on timestamps.
The situation
Two effects in the same layer (4, type-changing). Normally, timestamp order would decide — i.e. the order in which the cards entered the battlefield.
The dependency
If you apply Blood Moon first, Urborg becomes a Mountain and thereby loses all abilities — including its Swamp-granting ability. So whether Urborg's effect exists at all depends on whether Blood Moon was applied before it: Urborg's effect is DEPENDENT on Blood Moon.
Dependency beats timestamp
With a dependency, the independent effect is applied first — no matter which card entered the battlefield first. Blood Moon first: Urborg is a Mountain with no abilities, its effect no longer exists.
The result
All nonbasic lands (including Urborg) are just Mountains and produce 
. No land is a Swamp. That's why the answer to "But my Urborg was there first!" is always: it doesn't matter.
Urborg, Tomb of Yawgmoth
Legendary Land Legendary Land — Mountain
“Each land is a Swamp in addition …” “Each land is a Swamp in addition …”
🔴 Blood Moon
613.1The values of an object’s characteristics are determined by starting with the actual object. For a card, that means the values of the characteristics printed on that card. For a token or a copy of a spell or card, that means the values of the characteristics defined by the effect that created it. Then all applicable continuous effects are applied in a series of layers in the following order:
613.4Within layer 7, apply effects in a series of sublayers in the order described below. Within each sublayer, apply effects in timestamp order. (See rule 613.7.) Note that dependency may alter the order in which effects are applied within a sublayer. (See rule 613.8.)
613.7Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.
613.8Within 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.