El manubrio y el espejo retrovisor están limpios.

Because the subject is plural: el manubrio y el espejo retrovisor = the handlebar and the rearview mirror. Two things joined by y normally take a plural verb, so Spanish uses están.

• está limpio = it is clean
• están limpios = they are clean

Spanish usually uses estar for conditions or states that can change, and cleanliness is treated as a state. So están limpios means they are clean in the sense of their current condition.

Using son limpios would usually sound more like they are clean by nature / they are clean ones / they tend to be clean, which is not the normal idea here.

Because it agrees with the whole subject, which is plural: the handlebar and the rearview mirror. In Spanish, adjectives usually agree in both number and gender with the noun or nouns they describe.

• one masculine thing: limpio
• one feminine thing: limpia
• more than one masculine or mixed group: limpios
• more than one feminine thing: limpias

Here the subject is plural, so the adjective must also be plural: limpios.

Because the nouns here are masculine:

• el manubrio
• el espejo retrovisor

Since the group is plural and masculine, the adjective becomes limpios.

Also, in Spanish, if a group includes both masculine and feminine nouns, the default plural adjective is usually masculine plural.

Spanish often repeats the article before each noun in a list, especially when talking about specific items. So el manubrio y el espejo retrovisor is the natural way to say the handlebar and the rearview mirror.

You usually would not say el manubrio y espejo retrovisor here.

Manubrio usually refers to the handlebar of a bicycle or motorcycle in much of Latin America. Even though English often says handlebars in the plural, Spanish commonly uses the singular el manubrio for the whole bar assembly.

So:

• el manubrio = the handlebar / handlebars

Regional vocabulary can vary. In Spain, manillar is often more common.

It is a noun phrase made of:

• espejo = mirror
• retrovisor = rear-view / used for seeing behind

Together, espejo retrovisor means rearview mirror or rear-view mirror.

In everyday speech, people may sometimes shorten it to just retrovisor if the meaning is clear.

Because the sentence is talking about one mirror: el espejo retrovisor. If it were talking about more than one, it would become plural:

• el espejo retrovisor = the rearview mirror
• los espejos retrovisores = the rearview mirrors

Here, only one mirror is mentioned.

Yes, if the context is clear. Spanish often drops the subject when it is already known.

So if you were already looking at the handlebar and mirror, someone could simply say:

• Están limpios. = They are clean.

But the full sentence is useful when you want to name exactly what is clean.

Not in this sentence as written. Limpios is being used after estar as a predicate adjective:

• El manubrio y el espejo retrovisor están limpios.

If you put the adjective before the nouns, you would be building a different structure, and it would not sound natural in the same way.

The normal pattern here is:

• subject + estar
  • adjective

Most likely a bicycle or motorcycle, because manubrio strongly suggests a handlebar. A car would normally have a volante instead of a manubrio.

So the sentence naturally sounds like it is describing parts of a bike or motorcycle.

Then both the verb and adjective would become singular.

Examples:

• El manubrio está limpio. = The handlebar is clean.
• El espejo retrovisor está limpio. = The rearview mirror is clean.

The original sentence is plural because it talks about both items together.