Learn Hardcore German: Meine Freundin schwärmt von dem Sternenhimmel über dem See. - My girlfriend raves about the starry sky above the lake.

Freundin is feminine singular.
In this sentence, Freundin is the subject, so it is in the nominative case.
The nominative feminine form of mein is meine.

So:

mein Freund (masculine nominative)
meine Freundin (feminine nominative)

Does Freundin mean female friend or girlfriend here?

Freundin literally means female friend, but in everyday German it very often means girlfriend (romantic partner), especially when you say meine Freundin without extra context.

Context decides:

Das ist eine Freundin von mir. – That’s a (female) friend of mine.
Das ist meine Freundin. – That’s my girlfriend. (very likely romantic)

In your sentence Meine Freundin schwärmt …, many native speakers would automatically interpret meine Freundin as my girlfriend, unless the context clearly says otherwise.

What exactly does schwärmen mean, and how is it used?

schwärmen (in this sense) means something like:

to rave about something
to gush about something
to be enthusiastic / ecstatic about something

Typical pattern:

schwärmen von + Dativ – to rave about something

Examples:

Er schwärmt von seinem neuen Auto. – He raves about his new car.
Sie schwärmt von diesem Film. – She is crazy about this film.

There is also für etwas / jemanden schwärmen (to have a crush on someone / to be crazy about something), but that’s a slightly different nuance (more emotional, often romantic or fan-like).

Why is it schwärmt von and not schwärmt über?

In German, many verbs have a fixed preposition, and using a different one often sounds wrong or changes the meaning.

von etwas schwärmen = to rave about / be enthusiastic about something (this is the standard idiom)
über etwas schwärmen is not idiomatic in this meaning and sounds wrong to native speakers.

So you should learn schwärmen von as a fixed phrase:

schwärmen von dem Sternenhimmel – to rave about the starry sky

Why is it von dem Sternenhimmel and not another case?

The preposition von always takes the dative case.

Sternenhimmel is masculine: der Sternenhimmel.
Dative masculine singular of der is dem.

So:
von + dem Sternenhimmel → von dem Sternenhimmel (dative masculine)

You could also contract it in everyday language:

vom Sternenhimmel (von dem → vom)

What is the difference between von dem and vom here?

vom is simply the contracted form of von dem:

von dem Sternenhimmel = vom Sternenhimmel
von dem See = vom See

Both are grammatically correct; vom is more common in spoken and informal written German. Your sentence could very naturally be:

Meine Freundin schwärmt vom Sternenhimmel über dem See.

Why is it über dem See and not über den See?

über is a two-way preposition (Wechselpräposition). It can take either:

dative → when there is no movement / location, just where something is
accusative → when there is movement / direction, going to or across something

In your sentence, the starry sky is simply located above the lake (no movement), so we use the dative:

der See (nominative, masculine)
dem See (dative, masculine)

So: über dem See = above the lake.

Compare:

Der Vogel fliegt über den See. – The bird is flying across / over the lake (movement → accusative).
Die Wolken hängen über dem See. – The clouds are above the lake (location → dative).

Why is dem used twice – in von dem Sternenhimmel and über dem See?

Both Sternenhimmel and See are masculine nouns, and in this sentence both are in the dative singular:

von always takes dative:
- der Sternenhimmel → von dem Sternenhimmel
über takes dative when describing location (no movement):
- der See → über dem See

So you see dem twice because:

von + dative → von dem Sternenhimmel
über + dative (location) → über dem See

Is Sternenhimmel really one word? Why not Sternen Himmel?

Yes, Sternenhimmel is correctly written as one word. German very often creates compound nouns by joining words:

Stern (star) + Himmel (sky) → Sternenhimmel (starry sky)

Writing Sternen Himmel as two words is incorrect in standard German. Compounding is extremely common:

Haustür (house + door)
Bahnhof (train + yard/station)
Sonnenaufgang (sun + rise)

Also note:

Sternenhimmel is masculine: der Sternenhimmel.

Does über dem See describe the friend or the starry sky?

über dem See logically and grammatically describes the starry sky, not the friend.

The structure is:

von dem Sternenhimmel [über dem See]

So what is above the lake? The Sternenhimmel (the starry sky).
The meaning is: She raves about the starry sky that is above the lake, not My friend is above the lake.

If you wanted to say that the friend is above the lake (for example, in a helicopter), you’d have to change the word order or wording to make that clear, e.g.:

Meine Freundin ist über dem See und schwärmt vom Sternenhimmel.

Can I move the prepositional phrases around, like: Meine Freundin schwärmt über dem See von dem Sternenhimmel?

Grammatically, you can move prepositional phrases, but you must be careful about meaning and naturalness.

Meine Freundin schwärmt von dem Sternenhimmel über dem See.
→ Natural. Clearly: she raves about the starry sky above the lake.
Meine Freundin schwärmt über dem See von dem Sternenhimmel.
→ Grammatically possible, but sounds odd. It now suggests she is physically over the lake (e.g. in a boat in the middle), and there she raves about the starry sky. Most natives wouldn’t say it this way.

In general, keep von dem Sternenhimmel close to schwärmt von, because they belong together; then add extra details like über dem See after that noun phrase.

Is schwärmt here like English “is raving” or “raves”?

German doesn’t have a separate -ing form for continuous actions. The simple present (schwärmt) can express:

a general tendency:
Sie schwärmt von dem Sternenhimmel. – She raves about the starry sky (in general, whenever she talks about it).
an action happening right now (if context makes that clear):
In the right context, it can mean She is raving about the starry sky (right now).

So schwärmt can correspond to both English “raves” and “is raving”, depending on context.