Since .rbxl is a proprietary binary format, you cannot open it in a standard text editor like Notepad; it will simply appear as unreadable gibberish. Editing an RBXL file - Developer Forum | Roblox
Below is a blog post guide to understanding and using this specific file type. Scripts.rbxl
: Lighting, terrain, and camera positions. Game Services : Configuration for teams, chat, and more. Why name it "Scripts.rbxl"? and more. Why name it "Scripts.rbxl"?