how do i manage storage on mac version 10.10.5

First, if you are on 10.10, what model/year is your device - just want to make sure it is compatible with Big Sur.

Second, if you are on 10.10, you will need 45 GB of available space on your hard drive. (Having 10.12 Sierra installed, reduces it to 35.5 GB).

https://support.apple.com/kb/sp833?locale=en_US

So, get an external hard drive and move all of your space hogging files (movies, videos, photos,etc) to the external. After making sure they work, you can delete them on your internal.

You may want to add this to your bookmark collection - it is from Apple and explicitly states that the "other" category is managed by the OS and cannot be managed by the user (which is unfortunate at best):

https://support.apple.com/en-gb/guide/system-information/syspf9b375b9/11.0/mac/11.0

See used and available storage space on your Mac

How to free up storage space on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

@babowa - think that was the first link but thanks anyways :-)

Ahh, missed that - you are right!