How to Create OS Big Sur Bootable USB or SSD External Drive

I have an iMac 5K (27-inch, 2017) and a Macbook Air (13.3-inch, Mid 2013) both hard drives have failed. I would like to use an external SSD as the primary drive on both running MacOS Big Sur 11.7.10

I searched for how to do this and read the following: “Download from a Mac that is compatible with the macOS you're downloading. (If you’re downloading an older macOS, you might need to use an older Mac that is compatible with it.)”

How can I download and install that OS software on my external SSD if the only working Mac I have is running the latest Mac OS: Sonoma 14.5, and it will not only allow me to download and install an older version. If I can, will someone please explain how I can do this? Or can I purchase a bootable USB or External SSD with MacOS Big Sur? Remember when we had CD/DVD Drive and were given CD/DVD’s with the opeating system on them? I miss those days.