Is the Internal Drive the tiny 128 GB variety ?
If so, Apple should have never made that available at time of Purchase. It is almost useless for any meaningful usage IMHO.
How may upGrades have been performed since getting the computer ?
Did the computer come with Mojave and upGraded to Catalina and then Big Sur ?
The accumulated effects of several upgrades will take its toll on the available Empty Space available on the drive.
What is “Other” storage on a Mac, and how can I clean it out?
OmniDiskSweeper Safe to use
GrandPerspective
How to delete Time Machine snapshots on your Mac
See used and available storage space on your Mac
Locate backups of your iPhone, iPad, and iPod touch
The final word from Apple on Managing the " Other " Category
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.