Hi AGinCO,
Thanks for getting back to us with those additional details. We understand wanting to free up some space on your Mac so you can install the latest update. Let's continue working on this together.
Which macOS version is currently installed on your Mac? Apple > About This Mac.
Do you have multiple user accounts set up on your Mac? If so, have you tried to apply these tips to other user accounts as well?
If you go to: Apple > About This Mac > Storage. What seems to be taking up most of your storage? For example: photos, music or something else.
Please check this out:
macOS Big Sur - Technical Specifications
If you need further assistance, please contact Apple Support:
Get Support
Thank you for using Apple Support Communities. Have a nice day.