Magic byte is nothing special for Bitcoin, it's just a traditional name in binary data format identification. So everything in Bitcoin transaction begins with this magic byte, everything in a PNG file begins with this magic byte.
Bitcoin transactions don't begin with any magic byte. They start with a version number (hardcoded to 0,1,2) and optionally a 2-byte flag for segwit transactions.
Magic bytes are more used for files since most of the file formats are easily identifiable by their signatures at the beginning (the exception being different kinds of text files such as plain text vs code). But in any case, MIME types should be used for identification instead as they are superior to magic bytes since they can be stored as metadata.
