There is really nothing to be confused about. ㅃ is just another consonant like ㅂ or ㅍ is. You can't break it apart or create it by merging two ㅂs. 예뻐요 is always spelled that way and never as 옙버요. It is a fixed spelling of the word that you simply have to accept and memorize. It is the same as taking "pretty" as "pretty", not questioning why it has two t's instead of one. That is just the way it is.