Learn Turkish: Sponsorlar tarafından desteklenen ihtişamlı gala sonunda başlıyor. - The magnificent gala supported by the sponsors is finally starting.

It is built from the verb desteklemek (to support). First, we add the passive suffix -n to make desteklenmek (to be supported). Then, we add the subject relative participle -en to turn it into the adjective desteklenen (which is supported).

Why do we use tarafından here to say "by the sponsors"?

In Turkish passive sentences, the agent (the one actually doing the action) is expressed using the word tarafından (by). While you might sometimes see the suffix -ca or -ce used for "by", tarafından is the standard and most natural way to introduce a specific agent like Sponsorlar.

Does sonunda mean "at its end" or "finally" in this context?

Here it means "finally." Although sonunda literally translates to "at the end of it" (son + possessive -u + locative -nda), it is very commonly used as a standalone adverb meaning "finally" or "at last" to show that an anticipated event is happening.

Why is the phrase Sponsorlar tarafından desteklenen placed at the very beginning of the sentence?