This is the key. And don’t presume that once you’ve made a bootable installer on a thumb drive that you’ll just be able to pull it out after (say) 3 years to reinstall or install on another system. I had installers for three versions of MacOS, and very confidently tried to use one only to discover that the security certificate for it had expired. I had to do the work of preparing the thumb drive all over again, including the command line mumbo-jumbo.