The /personal folder is ideal to add ‘elements’ to the iso (and as a result to your future system). You can use them later after installation.
In this case I made the video to add a Mac driver to the folder. You can add files and folders to the airootfs but we recommend you to add packages to the /personal folder and install it lateron. The advantage is that pacman is control of that package and it can be updated automatically with the rest of the packages.
You can create a local repo and install a package that way to the iso. That is explained on https://alci.online.