you can use the word bank

can install K keyboard in settings

I think these are set spellings, even for loan words. Like we say 'baguette' and you couldnt spell it as 'bagget' or 'bagwet' or whatever, even though it sounds the same or very similar. Still a learner so not sure though :)

It does, This is the hangul for letter "M": ㅁ

That's because it is Japanese. The word 아파트 comes from Japanese "apato."