"Where is the house?"
Im new, so this might be wildly wrong and correct me if it is.
Gramatically, there is a difference between a 'topic' and a 'subject'. At its simpest, things happen to subjects, but things dont necessarily happen to topics.
For example, 'ジョンは大きいです', or 'John the topic big is' (John is big). In this sentence, John isnt doing anything, nor is anything happening to him.
However, if we say 'ジョンが食べた', or 'John the subject ate' (John ate), John is performing an action. Gramatically, John is no longer the topic, hes the subject, so you use が instead of は.
は marks the topic and emphasize what comes after particle while が identifies the subject and emphasize what comes before the particle.
You could translate them to Xは = "As for X" Xが = "X is the one (that)"
So using the above sentence this would clumsily translate to: ジョンは食べます As for John, he eats ジョンが食べます John is the one that eat
The emphasis part can be most easily shown with italic
わたしはスパタクスです I am Spartacus わたしがスパタクスです No! I am Spartacus!
Also as a extra, the particle を (pronounced 'o') is marking the object, so using the above sentence about John it would become ジョンを食べます (I) eat John
"Ha" indicates the main subject of a sentence. In this case, the house is the thing you're talking about. A more literal translation would be "As for house, where?". But, when you want to say "I'm looking for the house", you would use "ha" after "I" and "ga" after "house", and the sentence would translate to "As for me (subject), house (object) is the thing I'm looking for".
You can use が with です. Somtimes they even go together by default. There are fixed structures for general descriptions or for specific adjectives.
Elefants have big noses. (all of them as a characteristic) ぞう[は]はな[が]おおきいです。
I like the pen. ペン[が]すきです。
I hate insects. むし[が]きらいです。
There are many people. ひと[が]おおいです。
The addition of お is a politeness modifier, also known as an "honorific prefix", and it's not limited to topics or questions - you can add it to nouns* no matter how or where they're used.
*You can't just this to any noun. Some nouns use ご, which is a different reading of the same kanji as お (御), and some sound very strange with honorific prefixes at all.
There are two main reasons why you would add an honorific prefix:
1) to show respect to the owner of or the person associated with the thing you added お to (considered to be 尊敬語【そんけいご】"respectful language"), or
2) to show respect to the person you are speaking to, by speaking "politely"/"nicely"/"properly" (considered to be 美化語【びかご】"beautification language", which is a subset of 丁寧語【ていねいご】"polite language").
Both of your examples tend to fall under the first category; you would generally only use おうち or お弁当 to refer to someone else's house or lunch, respectively. Common examples of the second category are お茶【おちゃ】and お風呂【おふろ】- you aren't necessarily referring to someone else's tea or bathtub, but because these words hold significant importance in Japanese culture, it sounds improper to say them without the お.
Its both. Many Kanji have multiple pronunciations (multiple spellings) that potentially have different meanings. For example the Kanji for 7, or Minutes which was covered in a prior lesson. えい is to house as うち is to my house, but both use the same kanji despite the different spellings and pronunciations.