Learn Turkish: Şirin mi şirin bir vadi, heybetli dağların arasında uzanıyor. - A very cute valley extends between the majestic mountains.

This is a common and expressive way to intensify an adjective in Turkish! By repeating the adjective with the question particle mi in the middle (Adjective + mI + Adjective), you emphasize it heavily. It means "very cute" or "cute as can be." Even though the question particle is used here, this is a statement, not a question.

Why does bir come after şirin mi şirin rather than at the beginning of the sentence?

In Turkish, when a noun has both an adjective and the indefinite article bir, the standard word order is Adjective + bir + Noun. Treating the intensified phrase şirin mi şirin as a single adjective block, the correct order becomes şirin mi şirin bir vadi (literally: "very cute a valley").

I learned that uzanmak means "to lie down" or "to reach". How does it mean "extends" here?

You remembered perfectly! While uzanmak often means a person is lying down or reaching for something, it is also the standard verb used to describe geographical features, roads, or shadows that "stretch out" or "extend" across an area. Here, the valley is "lying" or extending between the mountains.

Why is the genitive ending used on the mountains (dağların arasında) instead of just saying dağlar arasında?

Using the full genitive-possessive construction (dağların arasında) makes the mountains specific or definite, meaning "between the majestic mountains" (referring to a specific mountain range). If it were dağlar arasında, it would mean "between mountains" in a general, non-specific sense.