how do i change core application icons?

I am running 10.15.7 on a mac mini from 2012, i can't update any further. I have been adding custom icons for all my apps, however, when I try to change the icons for the core apps (i.e., system preferences, app sore, notes, photos, messages, etc.) it doesn't let me, when I follow the step by step tutorial from apple, (Change icons for files or folders on Mac – Apple Support (AU) ) using the method apple suggested (copy and pasted buttons in the top bar) but the paste button is greyed out. I need any other strategies to get around the restrictions apple has placed. I am willing to try using the terminal if possible, however, keep in mind I am using 10.15.7 so it may be different. I have downloaded all my icons from macosicons.com.

I even tried entering the package contents section thru the applications tab in finder, but I can't add my own images into it, nor remove the current .icns that are in there.

any solutions??

p.s I have heard people say to use "csrutil disable" but I have no clue what that is, or how to use it