How can I delete unnecessary memory usage on my Mac?

Don't confuse storage and memory (RAM), they are two different things.

“System Data” (or “Other”) storage is an amalgam of files that don’t easily fall into the defined categories of “Audio" “Movies” “Photos” “Apps” and “Backups”.

You can’t directly manage the contents of this category. That is done by macOS. The category varies in size depending on the current state of your Mac.

What you actually can do…

• Empty the Trash in the Dock. 

• Empty the trash in the Photos app.

• Delete unused and unneeded application installers from your downloads folder and desktop.

• Reboot your Mac at least weekly.

• Transfer files that you don’t use daily to an external drive and then delete them from the startup drive and empty the trash. Files that take up the most room are movies, images and music.

This from Apple on the subject of freeing up space:

Free up storage space on Mac - Apple Support

Also, please see the following guidance from Apple: 

Find and delete files on your Mac - Apple Support

if you find you are running so short of space that you are tempted to use "Optimize Storage' feature to get 'free' space on iCloud to store your files, think VERY carefully. This feature is like a narcotic. Once you start to use it, it is very difficult to stop, and the more you use it, the more inconvenient it becomes, but you can't stop.

The real solution for many people is to add an external drive.

There are some very small and convenient ones.