In den Haferbrei kommen heute Rosinen und etwas Zimt.

Because in can take either the accusative or the dative, depending on the meaning:

• accusative = movement or direction into something
• dative = location inside something

Here, the raisins and cinnamon are being added into the porridge, so German uses in den Haferbrei.

Compare:

• In den Haferbrei kommen Rosinen. = raisins go into the porridge
• Im Haferbrei sind Rosinen. = there are raisins in the porridge

So den is there because Haferbrei is masculine and in takes the accusative here.

Because this is a normal main clause in German, and in German main clauses the conjugated verb must be in second position.

The first position is taken by In den Haferbrei, so the verb comes right after it:

• In den Haferbrei | kommen | heute Rosinen und etwas Zimt.

This is often called verb-second word order.

If the subject came first, the sentence could also be:

• Rosinen und etwas Zimt kommen heute in den Haferbrei.

That version is also correct, but it gives a slightly different emphasis.

The subject is Rosinen und etwas Zimt.

You can tell because that is what is doing the coming in the sentence. Also, the verb is kommen, which is plural, matching the compound subject:

• Rosinen
  • etwas Zimt = two things joined by und
• so German uses the plural verb kommen

Even though Zimt by itself is an uncountable noun, the whole phrase Rosinen und etwas Zimt is treated as a plural subject.

Because the subject is plural:

• Rosinen = plural
• Rosinen und etwas Zimt = a combined subject, so plural overall

So German uses:

• kommen = they come

If there were only one singular subject, you would use kommt:

• Etwas Zimt kommt in den Haferbrei.

Because German often uses bare plurals when talking about an indefinite amount of something plural.

So Rosinen here means something like raisins or some raisins, without needing an article.

Compare:

• Rosinen = raisins / some raisins
• die Rosinen = the raisins, a specific set already known
• einige Rosinen = some raisins, more explicitly

In recipe-style language, leaving out the article is very natural.

Because Zimt is normally treated as an uncountable noun here, like cinnamon in English.

For uncountable nouns, German commonly uses words like:

• etwas Zimt = some cinnamon
• ein bisschen Zimt = a little cinnamon
• viel Zimt = a lot of cinnamon

But ein Zimt would usually sound wrong in this context, because it would suggest one cinnamon as if it were a countable item.

Heute is a time adverb meaning today, and its position is fairly flexible.

In this sentence:

• In den Haferbrei kommen heute Rosinen und etwas Zimt.

it sits after the verb and before the subject, which is natural German.

You could also say:

• Heute kommen Rosinen und etwas Zimt in den Haferbrei.
• Rosinen und etwas Zimt kommen heute in den Haferbrei.

All of these are possible, but they shift the emphasis a little:

• starting with heute emphasizes today
• starting with In den Haferbrei emphasizes what they are going into
• starting with Rosinen und etwas Zimt is more neutral

Yes, kommen usually means to come, but in German it is also commonly used in contexts like cooking to mean that something goes into a dish or gets added.

So in a recipe or kitchen context, kommen in can sound very natural:

• In die Suppe kommen Karotten.
• In den Teig kommt Zucker.

English often uses go into, are added to, or just add. German can express the idea with kommen very naturally.

Yes. A very straightforward version would be:

• Rosinen und etwas Zimt kommen heute in den Haferbrei.

That is probably the most neutral order for many learners.

The original sentence:

• In den Haferbrei kommen heute Rosinen und etwas Zimt.

is also completely normal, but it puts the focus first on where the ingredients are going.

So the difference is not correctness, but information structure and emphasis.