You are doing great.
I am fully hurried at the moment... So I am going off of memory. I may get some of these wrong, but that's ok - you can explore which will help you learn better.
Sidepane:
File system, Computer, - Look in Scalable - Devices
Home (user-home.svg), desktop - Look in Scalable- Places
Trash - Look in Scalable - Status
Yes, you are right that a different sizing convention is being used. It's not too big a deal, as the system will scale the icons as needed in many cases.
So, for the 128px directory, replace those files from the ones in Zorin from the 48(2x) (these are actually size 96px... Close enough, for now.)