The RNA is only a recipe for how to string amino acids together, they already float around in the cell (they are small, simple molecules, like bricks for a hous

The RNA is only a recipe for how to string amino acids together, they already float around in the cell (they are small, simple molecules, like bricks for a house to confusingly change the metaphor). Only a piece of DNA is copied at a time into RNA, like you would load only one program into memory and not the entire harddrive. The mechanism for where to start and stop is farily simple in simpler organisms - you look for a start and termination sequence ("codon") (like ASCII control characters). In more complex organisms like multicellulars, this is more complicated. There are also other types of RNA that do other things.