Operating system using 75% of my MacBook memory - how to delete downloaded but unusable MacOS Big Sur

Have you Empties the Trash yet - am not being funny - that would be the final step in that process.

What is “Other” storage on a Mac, and how can I clean it out? A good start to make space

How to free up Space - another to make space

• See used and available storage space on your Mac - 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.

OmniDiskSweeper A Trusted Developers to identify additional files to remove

A 128 GB SSD is going to be very difficult to manage space on, even if doing a Wipe and Clean Installation of Big Sur. Really suggest getting an external drive and move as much of the Video, Music, Pictures, TV Videos , Pictures and iPhone Backups Off the drive and also evaluate all Required Applications versus Unneeded Applications and Remove the unneeded one as per Developers Instruction.

Although the computer may Qualify for Big Sur, the system requirements for the full experience of the newest OS are demanding. Especially on older machines as has been discovered - empty space.

It really comes down to want versus need. 8 GB RAM is basically the minimum, as is about 256 GB drive. Yes, it can run on 128 GB drive but there will be a struggle to put anything else on the machine.

If really considering downgrading - kind of a catch 22. The software updates to Mojave may continue just to the end of this year. Catalina updates may continue for another 1 to 1 1/2 years. Though, Catalina like Big Sur Only supports everything in 64 Bit, applications, extensions, drivers and there is no workaround.