أخصائي البصريات هنا.

Because Arabic usually leaves out the verb to be in the present tense.

So:

• أخصائي البصريات هنا
• literally: The optician here
• naturally: The optician is here

This is completely normal in Arabic. If you wanted the past, then Arabic would use a verb, for example:

• كان أخصائي البصريات هنا = The optician was here

Because أخصائي البصريات is an iḍāfa construction, often called a construct phrase.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun determines whether the whole phrase is definite or indefinite

So here:

• أخصائي = specialist
• البصريات = the optics / optical field

Together:

• أخصائي البصريات = the optics specialist / the optician

Since البصريات has الـ, the whole phrase is definite.

Literally, it means something like specialist of optics or optics specialist.

But in natural English, that is often translated as:

• optician
• sometimes optometry specialist, depending on context

So the Arabic expression is slightly more literal than the usual English job title.

Yes, it looks like a regular feminine plural form, but here it functions as the name of a field: optics.

So you should not think of it as meaning something simple like visual things in this sentence. In this context, it refers to the professional or scientific area of vision/optics.

That is why:

• أخصائي البصريات sounds natural as a job title or professional description.

This is a nominal sentence in Arabic.

Its two main parts are:

• أخصائي البصريات = the subject/topic (mubtadaʾ)
• هنا = the predicate/comment (khabar)

So the sentence structure is basically:

• [The optician] [is here]

Arabic often builds present-tense sentences this way, without an explicit verb to be.

Yes, هنا can come first.

The sentence as written:

• أخصائي البصريات هنا
• The optician is here

You can also say:

• هنا أخصائي البصريات

That can sound more like:

• Here is the optician
• or The optician is here, with extra focus on here

So both are possible, but the word order can slightly change the emphasis.

The main difference is definiteness.

• أخصائي البصريات = the optician / the optics specialist
• أخصائي بصريات = an optician / an optics specialist

Why?

Because in an iḍāfa, the second noun controls whether the whole phrase is definite:

• البصريات with الـ makes the whole phrase definite
• بصريات without الـ makes it indefinite

You would use the feminine form:

• أخصائية البصريات هنا.

That means:

• The female optician is here

So:

• أخصائي = male specialist
• أخصائية = female specialist

The rest of the sentence stays the same.

A few useful ones:

• هنا is pronounced hunā
• In البصريات, the الـ is pronounced clearly as al-, because ب is a moon letter, not a sun letter
• The letter ص in البصريات is an emphatic s sound, deeper/heavier than ordinary س

So البصريات is pronounced roughly like al-baṣariyyāt.

Even if you do not master every sound immediately, noticing the emphatic ص and the clear al- will help your pronunciation a lot.