I am have a hard time downloading big sur 11.3.1

I am sorry you are having this problem. You don't say what the problem is. So, I will offer some generic advice for you. Are you using Wi-Fi to do the download? If so, it is often corrupted because it is just too much data for Wi-Fi. Better to use an Ethernet cable connection to your modem/router. If it is not nearby, consider temporarily moving the computer to do the download. If that is not possible, you may have to do several lengthy downloads via Wi-Fi to get one that is clean and is successful. And be patient, a download can sometimes take up to an hour, more or less, with another 45 minutes to load. I hope it goes well for you.