How do I gain more memory on MacBook Air. It has 8GB. Thanks

You are talking about two separate things:

One is the memory, referred to as RAM. You have 8 GB and cannot add any on your model. That RAM is used to run your apps and everything you do. It has nothing to do with the space you need to an upgrade.

The other is your space on your hard drive/SSD. That is the place where your OS, system files, and all of your apps, videos, pics, and all other files are located. So, what is the capacity of your drive? And how much is used? You need to have at least 10 - 15 GB of empty space at all times for the OS to function properly. For the Big Sur upgrade, you actually need 35.5 - 45.5 GB for the download and install. In order to get some space, get an external hard drive and move all your space hogging things over. Then you can delete them on the internal and empty the trash until you've created enough empty space.

In order to check on your hard drive size, click on the Apple > about this mac > storage and post back with the numbers. Screenshot will work.

Again, it isn't memory - the documents, files, music, videos, movies, photos take up space on your hard drive/SSD. And, the easiest would be to get an external hard drive and move all of those files to it. Making this more difficult is the fact that you have an extremely small drive - 121 GB is tiny by today's standards. So you have to be very very conservative in what you want to download and install.

You cannot upgrade memory on a MacBook Air.