The confusion might be around this part in the guide
Then download the 19 installer parts for System 7.5.3 from Apple and place them in the folder.
Take a look at the file names. If the first is named with a ”.smi” extension and the others with a ”.part” extension, you can skip the next step of double-clicking all files individually. Double-clicking the files will unpack them, leaving you with the double amount of files in the folder. When finished, remove the files not named with the ”.smi” or ”.part” extensions.
It means that when the files you downloaded already have the one .smi and 18 .part extensions, you do not need to open them in OSX. They can be used in basilisk directly lateron in the installation.