Eski pasajın tavanında muhteşem vitray pencereler var.

-ın is the genitive suffix in Turkish. When you want to express “of something” (possession), the owner noun takes a genitive ending.
• pasaj → pasaj + ın = pasajın (“of the passage”)

The word tavan (“ceiling”) carries:
1) -ı (3rd-person singular possessive) → tavanı = “its ceiling”
2) -nda (locative) → tavanında = “on its ceiling”

So Eski pasajın tavanında literally means “on the old passage’s ceiling.”

var is the existential verb meaning “there is / there are.” In Turkish existential sentences the pattern is:
(locative) + (subject) + var/yok.
Here, muhteşem vitray pencereler is the subject, and var tells you that they exist.

In Turkish, when a noun modifies another noun (like an adjective), it stays singular.
• vitray modifies pencereler, so it remains vitray (“stained glass”)
• pencereler takes the plural suffix -ler to become “windows”

muhteşem is an adjective meaning “magnificent.” Turkish adjectives always precede the noun or noun phrase they describe, so it goes before vitray pencereler.

The typical pattern for existential sentences is indeed:
1) Locative phrase (where?)
2) Subject (what?)
3) var/yok (is/are)

You can shift elements for emphasis, but starting with the locative is the most natural way to set the scene.

A pasaj in Turkish is a covered shopping arcade or gallery—shops line a roofed walkway. It’s a loanword (from French passage), but it refers specifically to that architectural feature, not to a clause of text.