Quote from Gottschall >>

Can a sentient object deny nystul's magic aura like an unwilling character?

According to the RAW here, no, though sentient objects are pretty rare.

Let's say there's a magical item that has charges that can be used with a magical action, and it's under the effect of that spell and appears to be a non-magical item.
Can a character (especially a spellcaster) holding that item use the charges of that item if they think it's a non-magical item?
Can they understand/feel that the item has magical charges just by holding it?
Should the DM tell the character that they can use some magical action while holding that item (without other details)?

Quote from ZmeiyGames >>

Let's say there's a magical item that has charges that can be used with a magical action, and it's under the effect of that spell and appears to be a non-magical item.
Can a character (especially a spellcaster) holding that item use the charges of that item if they think it's a non-magical item?
Can they understand/feel that the item has magical charges just by holding it?
Should the DM tell the character that they can use some magical action while holding that item (without other details)?

To answer these questions in order:

1. No, you can't use the charges of a magic item you don't know is a magic item.
2. No, you can't feel that an item has magical charges just from holding it (though you can't do that anyway, so this spell has no impact on it).
3. No, the DM should not tell the character they can use a magical action while holding that item (though again, they shouldn't do that anyway).

Keep in mind, though that all this spell does when used in that manner is prevent spells and magical effects from identifying the object as magical. It doesn't stop someone from telling you the item is magical, attuning to it, or using it once you know what it is.

No, because when a creature dies it stops being a Creature and the body becomes an Object

Quote from EmperorManison >>

No, because when a creature dies it stops being a Creature and the body becomes an Object

Dead creatures are still creatures.

I couldn't find a clear information about my Question anywhere.

If I change my Creature-Type from Human to Undead would that mean that I become immune to the Poisoned, Charmed and Paralyzed Condition for example?

Those Creature Types have "in general" characteristic Immunities and/or Resistances accross the board. Would those also apply?

Is there anywhere a List where I could look up those changes or what creature-type brings certain benefits and/or debuffs?

Quote from Indyday >>

I couldn't find a clear information about my Question anywhere.

If I change my Creature-Type from Human to Undead would that mean that I become immune to the Poisoned, Charmed and Paralyzed Condition for example?

Those Creature Types have "in general" characteristic Immunities and/or Resistances accross the board. Would those also apply?

Is there anywhere a List where I could look up those changes or what creature-type brings certain benefits and/or debuffs?

No, because creature types do not automatically confer any immunities or resistances. While it's true that undead often have certain immunities and resistances, these are not universal to all undead, and they're due to these creatures' stat blocks explicitly saying that they're immune/resistant to certain things, not because of their creature type.

There's no list of benefits/debuffs provided by creature types because there aren't any. As stated in the Player's Handbook Rules Glossary here, "the [creature] types don't have rules themselves".

Thanks a lot.

So beside some niche applications this spell seems most useful with the "Suggestion" + "Planar Binding" Spells on creatures that aren't immune to the "Charmed" Condition to create several Pets/Servants/Spies you can command?

- Use Suggestion on a creature and tell it to stand still for 8 hours and accept all spell-effects (Nystuls and Planar Binding doesn't break the Charmed Condition and therefore the Creature remains "willing").

- Change the Creature-Type to any Type that works with "Planar Binding".

- You get a servant for the duration of "Planar Binding". -> 366 days at 9th level

Even with a 6th or 7th level Planar Binding that can be quite powerful.

Or is there anything very important/powerful that I miss completely?

Changing a magical object into a non-magical object (at least its appearance) sounds nice but I can't imagine to use this Spell very often over the course of a campaign

It can also protect a creature against harmful spells that target a specific creature type (e.g. Charm Person) but yes, it's a pretty niche spell.

Quote from wagnarokkr >>

It can also protect a creature against harmful spells that target a specific creature type (e.g. Charm Person) but yes, it's a pretty niche spell.

Well, "Suggestion" is a pretty common spell in most builds (that have access to it).

A Wizard can switch spells out so that shouldn't be a problem to create an army of servants. A bard most likely also has Suggestion or anything like that prepared. So for spending two prepared slots even a Bard could make many "minions".

So it can be really powerful in that way.

Sadly you can't change a Creature-Typ into the Type "Object". That would be S-Tier :D