What exactly *is* Custom Content? How can I add it to my game and create it?

I recommend using The Sims Resource. On my computer (Windows 10) I clicked the download button for an item I like on TSR, and then I get a popup at the bottom of the screen, click "save". Once I've downloaded all of the CC I want, I open my downloads folder. I drag all of the CC and drop it in the mods folder, found under The Sims 4 folder. Hope this makes sense.