SIM-kortet sidder i telefonen.

Because SIM-kortet is the definite form: the SIM card.

In Danish, definiteness is usually added as an ending:

• et SIM-kort = a SIM card
• SIM-kortet = the SIM card

So instead of using a separate word like English the, Danish often attaches it to the noun.

For the same reason: telefonen means the phone.

• en telefon = a phone
• telefonen = the phone

The sentence is talking about a specific phone, not just phones in general.

That is one of the basic features of Danish grammar. In simple noun phrases, the definite article is usually a suffix:

• en bil → bilen
• et hus → huset
• en telefon → telefonen
• et kort → kortet

English uses a separate word, the. Danish usually adds the ending directly to the noun.

That is a very common learner question. Danish uses sidde much more broadly than English uses sit.

Here, sidder means something like:

• is sitting
• is placed
• is fitted
• is inserted
• is located in position

So SIM-kortet sidder i telefonen means the SIM card is physically in place inside the phone.

Danish often uses sidde for things that are fixed or positioned somewhere:

• Nøglen sidder i låsen. = The key is in the lock.
• Der sidder en plet på trøjen. = There is a stain on the shirt.
• Batteriet sidder i computeren. = The battery is in the computer.

So this is very natural Danish.

Yes, you could, and it would be understandable. But it is less specific and a bit less idiomatic in this context.

• er i telefonen = is in the phone
• sidder i telefonen = is in the phone and fitted/positioned there properly

So sidder gives a stronger sense that the SIM card is inserted where it belongs.

Because the SIM card is inside the phone, so Danish uses i = in.

• i telefonen = in the phone
• på telefonen = on the phone

You would use på for something on the surface, like:

• Der er en ridse på telefonen. = There is a scratch on the phone.

But a SIM card is inside, so i is correct.

In Danish, nouns are either common gender or neuter.

Here:

• SIM-kort is neuter
  • et SIM-kort
  • SIM-kortet
• telefon is common gender
  • en telefon
  • telefonen

You can see the difference in both the indefinite article (en/et) and the definite ending.

Because SIM is an abbreviation, and Danish commonly uses a hyphen when an abbreviation is the first part of a compound noun.

So:

• SIM-kort
• tv-program
• EU-land

Then the definite ending is added to the whole noun:

• SIM-kortet

This makes the word easier to read.

The sentence follows normal Danish main-clause word order:

• SIM-kortet = subject
• sidder = verb
• i telefonen = prepositional phrase

So it is:

Subject + Verb + Place

This is the standard pattern in a simple statement.

A natural plural version would be:

SIM-kortene sidder i telefonerne.

That means the SIM cards are in the phones.

Breakdown:

• SIM-kort = SIM card / SIM cards
• SIM-kortene = the SIM cards
• telefoner = phones
• telefonerne = the phones

Danish often forms the plural definite by adding -ne.

Yes. sidder is the present tense of sidde.

• at sidde = to sit / to be positioned
• sidder = sits / is sitting / is positioned

In this sentence, it describes the current state of the SIM card: it is sitting in the phone now.

Because it is the thing the sentence is about, and it comes before the verb in a normal statement.

• SIM-kortet = the thing being described
• sidder = what it is doing / how it is positioned
• i telefonen = where it is

So the sentence is saying something about the SIM card.

Yes. It sounds natural and idiomatic.

A Dane would very naturally use sidder for a card, battery, key, screw, button, or similar object that is physically in place somewhere. So this sentence is good everyday Danish, not just textbook Danish.