I use the same notebook for every subject in college and code/date the lesson at the top. I have a folder for handouts, with each sheet coded with the same code and date so I know what belongs to what. I then have a separate ring binder for each subject and I separate and transfer the work over to the correct binder weekly. I just rip it out cause the paper comes out of my notebook cleanly. All my writing and any work sheets from a single lesson go into an individual plastic wallet in each folder so things from different lessons don’t get mixed up.