Do planes other than the Material detect as magic to Detect Magic?

Question

From my understanding of how the detect magic spell works, using it on another plane would cause problems. Primarily from my current understanding that:

• If a character used detect magic on another plane, everything would appear magical.
• Some of the planes have "effects" that use spells as their "effect", and following my previous point, everything would trigger the effect of the detect magic spell.
• To my knowledge, not all planes have these effects, but simply apply different physics and logic.

I am looking for a definitive source that would tell me whether everything not on the Material Plane is magical simply because it is from another plane of existence, and whether detect magic would indicate the same.

Answer 1

No, detect magic doesn't detect everything in another plane as magical

The Sage Advice Compendium addresses a related question:

Is the breath weapon of a dragon magical?

If you cast antimagic field, don armor of invulnerability, or use another feature of the game that protects against magical or non-magical effects, you might ask yourself, “Will this protect me against a dragon’s breath?” The breath weapon of a typical dragon isn’t considered magical, so antimagic field won’t help you but armor of invulnerability will.

You might be thinking, “Dragons seem pretty magical to me.” And yes, they are extraordinary! Their description even says they’re magical. But our game makes a distinction between two types of magic:

• the background magic that is part of the D&D multiverse’s physics and the physiology of many D&D creatures
• the concentrated magical energy that is contained in a magic item or channeled to create a spell or other focused magical effect

In D&D, the first type of magic is part of nature. It is no more dispellable than the wind. A monster like a dragon exists because of that magic-enhanced nature. The second type of magic is what the rules are concerned about. When a rule refers to something being magical, it’s referring to that second type. Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.

Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons are amazing, supernatural beings.

Detect magic, like other game mechanics, operates by this same logic with regard to what is considered magical. The general mystical effects of other planes are considered "the background magic that is part of the D&D multiverse's physics", unless explicitly described as magical or otherwise involving things already known to be magical (as described above).

Answer 2

There's some leeway in interpreting this, as per your source.

object in the area that bears magic

Bears magic is not a standard phrase, and so could be seen as detecting anything that could theoretically use magic. However, I've generally in tables seen it as detecting active supernatural powers or spells, not beings that contain magic or locations that contain magic.

That said, the aura detection is an active power. If you were in a magical location, you'd detect magic around you, but not see anything. Unless the plane was an object or a creature, you'd not see an aura around any effects. Spell like effects that are just floating around don't innately give off an aura. The ground often is not seen as an object.