InstallAssistant, a shell script to get the "InstallAssistant.pkg" and build "Install macOS XXX.app" for macOS you can even use it in the Recovery mode

installassistant -v -c https://swscan.apple.com/content/catalogs/others/index-26seed-26-15-14-13-12-10.16-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

list available versions

sh -c "$(curl -fsSL "https://raw.githubusercontent.com/sinojerk/installassistant/main/installassistant")"

list and with InstallAssistant.pkg url

sh -c "$(curl -fsSL "https://raw.githubusercontent.com/sinojerk/installassistant/main/installassistant")" -- -v

installassistant only find urls for you, you can download the "InstallAssistant.pkg" use your favorite tool after your "InstallAssistant.pkg" downloaded

sh -c "$(curl -fsSL "https://raw.githubusercontent.com/sinojerk/installassistant/main/installassistant")" -- build <path to your InstallAssistant.pkg>

will build the "Install macOS XXX.app" for you

1. reset your mac if needed, must go first(because your mac will reboot)
2. setup a ramdisk maybe 16GiB size

diskutil eraseDisk JHFS+ IA `hdiutil attach -nomount ram://$((16*1024*2048))`

3. cd /Volumes/IA then get a macOS version, download it, build to install macOS <xxx>.app see how to use section
4. run install macOS app (check the real path for install macOS <xxx>.app)

("/Volumes/IA/Applications/install macOS <xxx>.app/Contents/MacOS/InstallAssistant_springboard" &)

need to download InstallAssistant.pkg.integrityDataV1 file at the same time, the url is InstallAssistant.pkg file url by append .integrityDataV1