فيه ثوم وبصل في الصلصة، عشان كده ريحتها حلوة.

Here فيه is the very common Egyptian Arabic way to say there is / there are.

So:

• فيه ثوم وبصل = There is garlic and onion
• even though English uses there are with plural things, Egyptian Arabic often still uses فيه as a fixed existential expression.

A good way to think of it is:

• فيه = there exists / there’s

In fast speech, learners may hear it as fīh or sometimes almost just fī.

Because the two في forms are doing different jobs:

• فيه = there is / there are
• في الصلصة = in the sauce

So the structure is literally:

• فيه ثوم وبصل في الصلصة
• There is garlic and onion in the sauce

This is very normal in Egyptian Arabic.

Good question. فيه here is not referring to الصلصة directly. It is the fixed existential expression there is.

So:

• فيه ثوم وبصل في الصلصة = There is garlic and onion in the sauce

But if the sauce is already understood from context, you could say:

• فيها ثوم وبصل = It has garlic and onion / There is garlic and onion in it

Here فيها really does mean in it, with -ها referring to the feminine noun.

So:

• فيه = existential there is
• فيها = in it or it has, depending on context

Because they are being mentioned as ingredients in a general, indefinite way.

• ثوم = garlic
• بصل = onion

In English we also often say this without the:

• There’s garlic and onion in the sauce

If you said الثوم والبصل, it would sound more like the garlic and the onion, meaning specific garlic and specific onion already known in the conversation.

In Egyptian Arabic, ثوم is usually pronounced tūm or toom, not with the English th sound.

So although it is written with ث, the Egyptian pronunciation is commonly:

• ثوم → toom

This is a very common feature of Egyptian Arabic: some letters are pronounced differently from Modern Standard Arabic.

عشان كده means:

• that’s why
• for that reason
• so
• because of that

It connects the first idea to the result:

• there is garlic and onion in the sauce
• عشان كده
• that’s why its smell is nice

It is very common in spoken Egyptian Arabic.

You may also see:

• علشان كده

That is just a fuller spelling/pronunciation of the same expression.

ريحتها = ريحة + ها

• ريحة = smell / scent
• -ها = her / its for a feminine noun

So ريحتها literally means:

• its smell
• or more literally her smell, since Arabic uses the same suffix for feminine nouns

Here -ها refers to الصلصة, which is feminine.

So:

• ريحتها حلوة = Its smell is nice

Because it agrees with ريحة, which is a feminine noun.

• ريحة = feminine
• so the adjective is حلوة

If the noun were masculine, you would normally use حلو instead.

This agreement is very important in Arabic:

• feminine noun → feminine adjective
• masculine noun → masculine adjective

Here it means nice / pleasant, not literally sweet.

In Arabic, حلو / حلوة is very flexible and can mean things like:

• sweet
• nice
• pretty
• pleasant
• good

So ريحتها حلوة means:

• it smells good
• literally: its smell is nice

That is a very natural thing to say in Egyptian Arabic.

Because in Arabic, the present-tense verb to be is usually not stated.

So:

• ريحتها حلوة
• literally: its smell nice
• natural English: its smell is nice

This is completely normal in both Egyptian Arabic and Standard Arabic.

No. ريحة just means smell in general. It can be positive, negative, or neutral depending on context.

For example:

• ريحتها حلوة = its smell is nice
• ريحتها وحشة = its smell is bad

So the adjective tells you whether the smell is good or bad.

Yes, you could.

These are all natural, with slightly different structures:

• ريحتها حلوة = its smell is nice
• ريحة الصلصة حلوة = the smell of the sauce is nice
• الصلصة ريحتها حلوة = the sauce, its smell is nice / the sauce smells nice

The version in your sentence, ريحتها حلوة, is very natural because the sauce has just been mentioned, so the suffix -ها is enough.

It is clearly colloquial, especially because of عشان كده and the overall spoken style.

Some clues:

• فيه used this way is very common in speech
• عشان كده is colloquial
• ريحة is the everyday spoken word, where Standard Arabic more often uses رائحة

A more formal Standard Arabic version would be different, but this sentence is perfectly natural for everyday Egyptian speech.