"I'm sorry. Thank you."
송. As far as I know, every syllable contains must have at least one vowel in Korean.
You can also see that in some typical Konglish words.
As Korean does not have the sta combination, one has to put the s by itself. However, a vowel is required for every syllable, so very often the neutral ㅡ vowel is used, leading to 스.
It should be "song (ㅅ + ㅗ + ㅇ)".
Korean is written as blocks consisting of several characters and each block represents one syllable.
What you are referring to is the second character of the block and for any block, it should always be a vowel. Additionally, the first and third characters of a block should always be consonants as you can see with ㅅ and ㅇ.
This concept is better explained with visuals. Take a look here: https://www.howtostudykorean.com/unit0/unit0lesson1/
Like most of the questions on this thread, the primary difference is formality. "미안해요" is more informal and connotes a familiar "Sorry", with the word "I" not present at the beginning of the word. "죄송합니다" is more formal and includes "죄", which means "I", as in "I am sorry".
nleconte posted a good article on the different ways of saying I'm sorry in Korean in one of the other questions above, I've repeated it here for your convenience: