Questo mobile è ingombrante e diventa un rifiuto ingombrante.

In this sentence, mobile is a noun, and it means a piece of furniture.

• il mobile = the piece of furniture / the cabinet / the furnishing item
• mobile as an adjective can also mean movable or mobile
• mobile phone in Italian is usually cellulare or telefonino, not just mobile the way it is in British English

So in Questo mobile, mobile is definitely a furniture item, not a phone.

Because mobile meaning piece of furniture is a masculine singular noun.

So the demonstrative must match it:

• questo mobile = this piece of furniture
• questi mobili = these pieces of furniture

If the noun were feminine, you would use questa, but not here.

Ingombrante means something like:

• bulky
• cumbersome
• taking up a lot of space

So Questo mobile è ingombrante means the furniture item is physically large or awkward and occupies a lot of room.

It is a very common word in contexts like:

• furniture
• packaging
• waste disposal
• moving house

Some Italian adjectives have the same singular form for both masculine and feminine. Ingombrante is one of them.

Pattern:

• masculine singular: ingombrante
• feminine singular: ingombrante
• masculine plural: ingombranti
• feminine plural: ingombranti

So:

• un mobile ingombrante
• una scatola ingombrante
• due mobili ingombranti
• due scatole ingombranti

That is why it does not become ingombranto or anything like that.

The repetition sounds natural here because the word is doing two slightly different jobs.

1. Questo mobile è ingombrante
   Here it is a normal adjective describing the furniture item: it is bulky.

2. diventa un rifiuto ingombrante
   Here rifiuto ingombrante is almost a set expression, especially in public-service or recycling language. It means bulky waste.

So the second ingombrante is not just random repetition; it helps name the category the object belongs to once you throw it away.

They express two different ideas:

• è = is
  This describes a current quality.
• diventa = becomes
  This describes a change of state.

So:

• Questo mobile è ingombrante = This piece of furniture is bulky.
• diventa un rifiuto ingombrante = it becomes bulky waste.

The sentence moves from what the object is now to what it turns into in a disposal context.

Diventa is the third-person singular present indicative of diventare.

Conjugation:

• io divento
• tu diventi
• lui/lei diventa
• noi diventiamo
• voi diventate
• loro diventano

Here diventa refers to questo mobile, which is singular.

Because rifiuto is being presented as one item / one instance of that category.

• un rifiuto ingombrante = a bulky waste item / an item of bulky waste

Italian often uses the indefinite article when classifying something:

• È un problema.
• Diventa un rifiuto.
• È un oggetto fragile.

So un is completely natural here.

Not exactly.

• rifiuto = waste, refuse, often more formal, technical, or administrative
• spazzatura = trash/garbage, more everyday language

In recycling or municipal language, rifiuto is very common.
So rifiuto ingombrante is the kind of term you might see in:

• recycling instructions
• city waste collection websites
• rules for disposal of furniture or appliances

It sounds more official than spazzatura ingombrante.

In Italian, adjectives often come after the noun, especially when they are giving a straightforward description or classifying the noun.

So:

• rifiuto ingombrante
• mobile ingombrante
• oggetto pesante

That is the most normal order here.
Putting the adjective before the noun would usually sound unusual, literary, or would change the emphasis.

Yes. The plural of mobile is mobili.

Singular:

• Questo mobile è ingombrante e diventa un rifiuto ingombrante.

Plural:

• Questi mobili sono ingombranti e diventano rifiuti ingombranti.

Notice all the agreement changes:

• questo → questi
• mobile → mobili
• è → sono
• ingombrante → ingombranti
• diventa → diventano
• un rifiuto → rifiuti

Mobile can be a fairly broad word for a piece of furniture, but in real usage it often suggests a substantial furniture item such as:

• a cabinet
• a cupboard
• a unit
• a dresser

It is less likely to be used for every possible item in the broad English sense of furniture.
For furniture in general, Italian often uses:

• i mobili = furniture / furnishings
• arredamento = furnishing / decor / furniture as a category

So in this sentence, mobile sounds like a furniture item, probably one big enough to become bulky waste.

Yes, and the meaning would shift slightly.

• Questo mobile = this piece of furniture
  Refers to a specific one, probably near the speaker or already being pointed out.
• Il mobile = the piece of furniture
  Refers to a known or previously mentioned item.

So questo is used because the speaker is identifying a particular object directly.