Learn Turkish: Orası açık mı?

Orada means "in that place" or "over there" because it has the location ending. If you used it here, you would be asking "Is it open at that place?". By using orası, you make the location the actual subject of the sentence, so it translates perfectly to "Is that place open?".

What is the -sı ending on orası?

It is actually the 3rd-person possessive suffix! The base word is ora (there). Turkish adds -sı to turn these location words into specific noun subjects. You will see this pattern often, such as in burası (this place) and şurası (that place right over there).

Why is the question word mı instead of mi or mu?

The Turkish question particle always follows 4-way vowel harmony based on the last vowel of the word right before it. The last vowel in açık is ı, so the question particle must be mı.

Does açık only mean "open" for buildings and doors?

It means "open" for shops and doors, but it is also a very useful everyday word with other meanings! You can use açık to say that an electronic device like a TV or phone is "turned on", that the weather is "clear", or that a color is "light".