La correa está junto a la puerta.

Spanish normally uses estar for location:

• La correa está junto a la puerta. = the leash/strap is next to the door.

That is why está is used here.
Ser is usually for identity, characteristics, origin, etc.

Compare:

• La puerta es de madera. = The door is made of wood.
• La correa está junto a la puerta. = The leash is next to the door.

Even if the object is usually kept there, Spanish still uses estar for where it is located.

Junto a means next to, beside, or right by.

It usually suggests close physical position, often closer than cerca de (near).

So:

• junto a la puerta = next to the door
• cerca de la puerta = near the door

A very common alternative is:

• al lado de la puerta

That means almost the same thing.

Because junto a is the normal expression. The a is part of the structure that introduces what something is next to.

So Spanish says:

• junto a la puerta
• junto a la mesa
• junto al coche (if the next noun is masculine singular)

You generally should learn it as one unit: junto a.

Because both correa and puerta are feminine singular nouns, so they take la.

• la correa
• la puerta

In Spanish, articles have to agree with the noun in gender and number.

Also, Spanish uses definite articles more often than English does. English might sometimes say the or sometimes leave it out depending on style, but Spanish often keeps the article.

No, that would sound wrong in normal Spanish.

You usually need the articles here:

• La correa está junto a la puerta.

Without the articles, it sounds incomplete or unnatural. Since you are talking about a specific leash/strap and a specific door, la is the normal choice for both nouns.

Not always. Correa can mean different things depending on context, such as:

• leash
• strap
• belt
• drive belt or belt in a machine

In much of Latin America, if you mean the belt you wear with pants, cinturón is often more specific.
So correa often feels more like strap or leash, depending on the situation.

In this sentence, the context tells you which meaning is intended.

Yes. That is a very natural alternative.

• La correa está junto a la puerta.
• La correa está al lado de la puerta.

Both are correct and common.
If anything, al lado de may sound a bit more conversational in everyday speech, while junto a can sound slightly more neutral or a little more formal, depending on the speaker and region.

Not exactly.

• junto a la puerta = next to the door
• cerca de la puerta = near the door

Junto a usually implies a position very close, almost touching or directly beside.
Cerca de is less exact: it only means nearby.

So if the leash is on the floor right by the door, junto a is better.

Yes. If a is followed by el, they contract to al.

So:

• junto a la puerta
• junto al carro
• junto al sofá

You do not say junto a el carro in standard Spanish.
It becomes junto al carro.

Yes, but the original order is the most neutral:

• La correa está junto a la puerta.

You can also say:

• Junto a la puerta está la correa.

That version is grammatical, but it sounds more emphatic, literary, or like you are highlighting the location first.

For most everyday situations, the basic order is best:

subject + está + location

You would change the article, noun, and verb to plural:

• Las correas están junto a la puerta.

Changes:

• la → las
• correa → correas
• está → están

If there is more than one door too, then:

• Las correas están junto a las puertas.

Spanish requires agreement in number, so the articles and verb must match the noun.