I'm trying to figure out how points of interest work but I have no idea. It'd be cool if there was an option to just place them down on the map and name them, or a guide for dummies like me who don't know how JSON files work.

You can just place them down, but first you need to "enable" them: Style > Elements > Landmarks > Icon or Legend. After that you can right-click anywhere and there will be "Add landmark" in the context menu. I know it's not very obvious and I'll try to fix it. You only need JSON if you want to create a bunch of them in advance and try them on different maps.