tenin ha 「sityakusitu ha uketuke no yoko ni arimasu」 to iimasita.

They belong to two different layers of the sentence.

• The first は in 店員は marks the shop clerk as the topic of the whole sentence.
• The second は in 試着室は受付の横にあります is inside the quoted speech, where 試着室 is the topic of what the clerk said.

So the structure is basically:

• As for the shop clerk,
  they said:
• As for the fitting room,
  it is next to the reception desk.

The と after あります marks the content of a quotation before a verb like 言いました.

So:

• 「試着室は受付の横にあります」 = the quoted words
• と = marks that quote
• 言いました = said

This pattern is very common:

• A と言いました = said A
• A と思います = think A
• A と書いてあります = it says A

Japanese usually uses:

• あります for things that are not living beings
• います for people and animals

A fitting room, 試着室, is an inanimate place, so あります is correct.

Compare:

• 猫がいます = There is a cat.
• 机があります = There is a desk.
• 試着室は受付の横にあります = The fitting room is next to the reception desk.

It means next to the reception desk.

Breakdown:

• 受付 = reception, front desk
• の = links nouns, often like of or showing relation
• 横 = side
• に = location marker here

So:

• 受付の横 = the side of the reception desk / beside the reception desk
• 受付の横にあります = is beside the reception desk

Because に marks the location where something exists when used with あります or います.

In this sentence:

• 受付の横に = at the side of the reception desk
• あります = exists / is located

This is a standard pattern:

• 場所 に あります
• 場所 に います

Examples:

• 部屋に机があります = There is a desk in the room.
• 公園に子どもがいます = There are children in the park.

の connects nouns. Here it shows that 横 is related to 受付.

So 受付の横 literally means something like:

• the side of the reception desk
• or more naturally, beside the reception desk

This use of の is extremely common in Japanese:

• 学校の先生 = the school’s teacher / teacher at the school
• 机の上 = on the desk
• 店の前 = in front of the shop

The dictionary form is 言う and it is read いう.

When it changes to the polite past form, it becomes:

• 言う → 言います → 言いました

The pronunciation becomes いいました, with a long ii sound in effect.

This happens because the う in いう changes as the verb is conjugated. So although the dictionary form is いう, the polite stem is いい-.

Common forms:

• 言う = to say
• 言います = say / will say
• 言いました = said
• 言って = say (te-form)

Yes, mostly. They mark direct quotation.

So:

• 「試着室は受付の横にあります」 = The fitting room is next to the reception desk

Japanese commonly uses:

• 「 」 for normal quotations
• 『 』 sometimes for quotations inside quotations, titles, or emphasis

In this sentence, 「 」 simply shows the exact words the clerk said.

Yes, but the nuance changes a little.

• 店員は presents the clerk as the topic: As for the clerk, they said...
• 店員が would focus more specifically on the clerk as the subject: It was the clerk who said...

In many contexts, は sounds more neutral here, especially if the clerk is already part of the situation being talked about.

Yes. It is a complete sentence.

Structure:

• 試着室は = as for the fitting room
• 受付の横に = next to the reception desk
• あります = is located / exists

So it works perfectly on its own as a polite statement.

Then the larger sentence adds:

• と = quotation marker
• 言いました = said

Here are the common readings:

• 店員 = てんいん
• 試着室 = しちゃくしつ
• 受付 = うけつけ
• 横 = よこ
• あります = あります
• 言いました = いいました

A full reading of the sentence is:

てんいん は 「しちゃくしつ は うけつけ の よこ に あります」 と いいました。

This is direct quotation, because the exact words are presented inside 「 」.

Direct quotation:

• 店員は「試着室は受付の横にあります」と言いました。

Indirect quotation would be more like:

• 店員は試着室が受付の横にあると言いました。

In the indirect version, the quoted sentence is reshaped grammatically, and you usually do not use 「 」.

Using は makes 試着室 the topic: As for the fitting room, it is next to the reception desk.

That is very natural when giving directions or explaining where something is.

If you used が, it would sound more like identifying it or emphasizing it in a specific context:

• 試着室が受付の横にあります

That can also be grammatical, but 試着室は is more natural when the fitting room is already what the conversation is about.

Yes. Japanese often omits things that are clear from context.

For example, if everyone already knows you are asking about the fitting room, a clerk might simply say:

• 受付の横にあります。

That still means It’s next to the reception desk.

Japanese frequently leaves out subjects and topics when they are obvious. The full version in your sentence is just clearer for learners and perfectly natural too.