هذا مزود خدمة الإنترنت.

In Arabic, the verb to be is usually not expressed in the present tense.

So a sentence like هذا مزود خدمة الإنترنت naturally means This is the internet service provider without any written word for is.

If you wanted a past or future meaning, Arabic would normally use a verb, such as كان for was.

هذا means this and is the masculine singular demonstrative.

It is used here because the noun مزود is masculine singular.
If the referent were feminine, Arabic would usually use هذه instead.

مزود means provider or supplier.

It comes from the verb زوّد, meaning to provide / supply.

Its usual pronunciation is muzawwid. In fully vowelled Arabic, it is مُزَوِّد.

This is an iḍāfa construction, often called a construct phrase.

• خدمة = service
• الإنترنت = the internet

Together, خدمة الإنترنت literally means service of the internet, which in natural English becomes internet service.

Then the full phrase مزود خدمة الإنترنت means provider of internet service.

Because this is an iḍāfa chain.

In an iḍāfa:

• the first noun does not take الـ
• it also does not take tanwīn
• the definiteness of the whole phrase is determined by the last noun

So although مزود and خدمة do not have الـ, the whole phrase can still be definite because الإنترنت is definite.

Because خدمة is followed by another noun in an iḍāfa construction.

When a word ending in ة is in pause by itself, it is often pronounced like -ah:
khidmah

But when it is linked to another word grammatically, it is pronounced -at / -t:
khidmat al-internet

This is very common in Arabic.

This is a very good question, because the phrase can be ambiguous in unvowelled Arabic.

Since مزود خدمة الإنترنت is a definite noun phrase, هذا مزود خدمة الإنترنت can be read as:

• This is the internet service provider, or
• this internet service provider

If you want to make the sentence reading especially clear, Arabic often adds هو:

هذا هو مزود خدمة الإنترنت

That more clearly means This is the internet service provider.

In Modern Standard Arabic, الإنترنت is commonly used with الـ because it refers to a specific, unique thing: the internet.

So learners will often see:

• الإنترنت = the internet

In practice, borrowed words sometimes show spelling variation, but الإنترنت is very common and standard.

A fully vowelled version is:

هٰذَا مُزَوِّدُ خِدْمَةِ الْإِنْتَرْنِتِ

A careful transliteration would be:

hādhā muzawwidu khidmati l-interneti

Notes:

• مزودُ is nominative because it is the predicate
• خدمةِ is genitive because it is in iḍāfa after مزود
• الإنترنتِ is also genitive because it completes the next iḍāfa

In normal writing and everyday pronunciation, these case endings are often not shown.