Fuar alanında stand kuruluyor.

Because fuar alanı (“fairground”) is a compound where alan (“field/area”) belongs to fuar (“fair”), so you first add the 3rd-person possessive -ı to get alanı (“its field” = “fairground”) and then the locative -nda (“in/at”).
Formation:
• alan + -ı → alanı
• alanı + -nda → alanında (“in/at the fairground”)
If you just had alan by itself, you would get alanda for “in the field/area.”

Turkish applies case endings after possessive endings when a noun is part of a compound.
1) alan + -ı (3rd-person possessive, because it’s the “fair’s field”) → alanı
2) alanı + -nda (locative) → alanında
The order is always root → possessive → case.

kuruluyor is passive voice in the present-continuous (progressive) tense.
Breakdown:
• kur- (root “to set up”)
• -ul-/-ül- (passive suffix) → kurul-
• -uyor (progressive suffix) → kuruluyor
There is no personal ending shown because 3rd-person is zero-marked in this construction.

You add an agent with tarafından (“by”). For example:
Fuar alanında stand fuar organizatörleri tarafından kuruluyor.
= “The stand is being set up at the fairground by the fair organizers.”

In active voice you need an explicit subject (usually plural “they” when unknown):
Fuar alanında stand kuruyorlar.
= “They are setting up a stand at the fairground.”

• kuruluyor = “is being set up” (ongoing action, present-continuous)
• kuruldu = “was set up” or “has been set up” (completed action, simple past/passive aorist)

Turkish is fairly flexible. Your original Fuar alanında stand kuruluyor (Adjunct → Theme → Verb) is most neutral. You could also say:
• Stand fuar alanında kuruluyor. (Theme first)
• Kuruluyor stand fuar alanında. (Very marked, for emphasis)
But the standard is locative adjunct first, then the noun being set up, then the verb.