Did you actually install Big Sur or just downloaded the Big Sur installer?
If you only downloaded the Big Sur installer, then look in the Applications folder for the "Install macOS Big Sur" app and delete it & empty the Trash.
FYI, you should always have at least 20GB+ of Free storage space at all times for the normal operation of macOS. You may need even more Free storage space for some workloads since 20GB is the bare minimum & can disappear very quickly with some workloads. Ignore the "Available" storage value since it is very misleading. Unfortunately the Free space value is only listed in the inconvenient locations of Disk Utility and the System Profiler although it can be calculated as well if presented with the necessary values.