Das Haus sieht von außen schön aus.

Because the full verb is aussehen, which is a separable verb.

• Infinitive: aussehen = to look / to appear

• In a main clause, the conjugated part goes to the usual verb position, and the prefix goes to the end:

• Das Haus sieht ... aus.
• Ich sehe müde aus.
• Sie sieht gut aus.

So in this sentence:

• sieht = the conjugated verb part
• aus = the separable prefix

That is completely normal German word order for separable verbs.

Not by itself. The full verb is aussehen, and that means to look or to appear.

So:

• sehen = to see
• aussehen = to look / to appear

That is why Das Haus sieht von außen schön aus does not mean The house sees beautifully from the outside. It means the house looks beautiful from the outside.

Because Das Haus is the subject of the sentence, so it is in the nominative case.

The house is the thing that is doing the looking/appearing:

• Das Haus sieht schön aus. = The house looks beautiful.

So:

• das Haus = nominative singular
• dem Haus = dative singular

You would only use dem Haus if the grammar required dative, which it does not here.

Von außen is a very common expression meaning from the outside or externally.

Here:

• von takes the dative
• außen works as an adverbial expression meaning outside / the outside

So von außen means:

• from the outside
• seen externally
• from an exterior viewpoint

It is the natural German way to say this idea in this sentence.

Because German simply does not normally use aus außen for this meaning.

English learners often expect a more literal word-for-word match, but German uses the fixed expression von außen.

Compare:

• von außen = from the outside
• von innen = from the inside

These are standard expressions.

Because schön is a predicative adjective here, not an adjective directly in front of a noun.

Compare:

• das schöne Haus = the beautiful house
  Here schöne describes the noun directly, so it gets an ending.

• Das Haus ist schön.
• Das Haus sieht schön aus.
  Here schön comes after a verb and describes the subject, so it does not take an ending.

This is the same pattern as with:

• Das Auto ist teuer.
• Er ist müde.
• Sie sieht glücklich aus.

German main-clause word order puts the conjugated verb in the second position, and with a separable verb the prefix goes to the end.

So the frame is:

• Das Haus | sieht | ... | aus

Everything else goes in the middle:

• von außen = from the outside
• schön = beautiful

That gives:

• Das Haus sieht von außen schön aus.

This is very typical German sentence structure.

Yes. That means:

• The house looks beautiful.

Adding von außen makes the meaning more specific:

• Das Haus sieht schön aus. = general statement
• Das Haus sieht von außen schön aus. = it looks beautiful from the outside

The longer sentence can sometimes imply that we are only talking about the exterior, not necessarily the interior.

Often, yes.

When someone says:

• Das Haus sieht von außen schön aus.

it can simply mean the exterior is beautiful, but it may also suggest:

• maybe we are only judging the outside
• maybe the inside is different
• maybe we have not seen the inside yet

So the phrase can carry a mild contrast, depending on context.

Yes, but it sounds a bit different.

• Das Haus sieht von außen schön aus. = natural everyday phrasing
• Das Haus sieht äußerlich schön aus. = more formal or abstract, something like externally

For normal conversation, von außen is usually the more natural choice.

Because here außen is not being used as a normal noun. It is part of an adverbial expression.

German capitalizes nouns, but not ordinary adverbs. In:

• von außen
• von innen

both außen and innen are normally lowercase.

Then the full separable verb comes together at the end.

Main clause:

• Das Haus sieht von außen schön aus.

Subordinate clause:

• ..., weil das Haus von außen schön aussieht.

So:

• main clause: sieht ... aus
• subordinate clause / infinitive form: aussieht / aussehen

That is another very common feature of separable verbs in German.