Zum Nachtisch essen wir Joghurt mit Himbeeren und Blaubeeren.

It means for dessert or as dessert.

Literally, zu dem Nachtisch would be something like to the dessert, but in German zum Nachtisch is a normal idiomatic way to say for dessert.

So:

• Zum Nachtisch essen wir … = For dessert, we eat …

Zum is just the contracted form of zu dem.

• zu dem → zum

This contraction is very common in German and is usually what native speakers say.

Here, Nachtisch is masculine:

• der Nachtisch

And zu takes the dative case, so:

• zu dem Nachtisch
• contracted: zum Nachtisch

Because German main clauses follow the verb-second rule.

The first position in the sentence is taken by Zum Nachtisch, so the conjugated verb must come next:

• Zum Nachtisch | essen | wir | Joghurt ...

This is why you do not get:

• Zum Nachtisch wir essen ... ❌

German wants the finite verb in second position.

Yes. That is also correct.

Both are natural:

• Zum Nachtisch essen wir Joghurt mit Himbeeren und Blaubeeren.
• Wir essen zum Nachtisch Joghurt mit Himbeeren und Blaubeeren.

The difference is mainly emphasis:

• Zum Nachtisch ... puts for dessert first and gives it a little more focus.
• Wir essen ... starts more neutrally with we eat.

Because Joghurt is being used as a general food item / mass noun, not as a specific identified yogurt.

German often leaves out the article with foods in this kind of sentence:

• Wir essen Joghurt.
• Sie trinkt Kaffee.
• Er kauft Brot.

If you add an article, it usually sounds more specific:

• Wir essen den Joghurt. = we are eating the yogurt
• Wir essen einen Joghurt. = we are eating a yogurt / one yogurt

So in this sentence, no article is completely normal.

Mit always takes the dative case.

So:

• mit Himbeeren und Blaubeeren

is in the dative plural.

A useful thing to know: sometimes you cannot easily see the dative on the noun form, especially in the plural, because the plural form may already end in -n or -en.

Here:

• die Himbeeren → mit Himbeeren
• die Blaubeeren → mit Blaubeeren

The form looks the same, but the case is still dative because mit requires it.

Because the sentence is talking about them in a general, non-specific way.

• Joghurt mit Himbeeren und Blaubeeren = yogurt with raspberries and blueberries

This is similar to English, where you would usually not say with the raspberries and the blueberries unless you mean specific ones already known in the conversation.

If you wanted to make them specific in German, you could use articles:

• mit den Himbeeren und den Blaubeeren

But that would usually sound much more specific than the original sentence.

Because the sentence means yogurt with raspberries and blueberries in general, not just one raspberry and one blueberry.

Singular would be:

• die Himbeere = raspberry
• die Blaubeere = blueberry

Plural:

• die Himbeeren = raspberries
• die Blaubeeren = blueberries

In food descriptions, plural is very common when talking about fruit pieces or fruit as a topping.

No. A very common alternative is Heidelbeeren.

Both can mean blueberries. Which one people use can depend on region and context.

So these are both normal:

• Joghurt mit Blaubeeren
• Joghurt mit Heidelbeeren

For a learner, it is enough to recognize that both words can refer to blueberries.

Because they are nouns, and in German all nouns are capitalized.

So in this sentence:

• Nachtisch = noun
• Joghurt = noun
• Himbeeren = noun
• Blaubeeren = noun

This is a basic spelling rule in German and one of the easiest ways to spot nouns.