In short, you cannot learn Jiu-Jitsu entirely on your own or just online. The ground fighting style requires training partners and coaches for you to feel and learn the moves not just in your mind but with the mind-muscle connection. Even if you were to watch 10 hours of drills for basic Jiu-Jitsu online, stepping onto the mat would not give you 10 hours' worth of experience. What can be great is supplementing the moves and techniques you learn in class or with a coach. Using a dummy in this situation to practice hundreds of times will help solidify the technique and your body's ability to perform the move quicker and more instinctively. Online Jiu-Jitsu videos are also a fantastic way to learn new moves, experiment, and quickly find new techniques. Nothing is stopping you from performing these on your dummy, and eventually, on your training partners. Additionally, these videos are typically by advanced BJJ practitioners and will solidify your mind's technical aspects as they talk through the "why's" of the technique.