Big sur

Boot in Safe Mode again and go to applications folder and see if there is a " Install Big Sur " application. If Yes, Drag and Drop to Trash and empty Trash.

Still in Sade Mode - See used and available storage space on your Mac and what does it report as Empty Space

Realize this

• Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

> I cannot get to data files to delete or shift onto an external hard drive since the computer is in safe mode

At least in my 11.5.2 setup I can boot into safe mode and copy files to an USB flash drive. So you should be able to move large files to external drives (backup at least those files on two separate disks -- a complete backup is highly recommended) and then delete them from the internal disk. You need about 40-60 GB free space for the full Big Sur install.