Whether learning from online videos or getting instruction from a private teacher in piano classes, all students who want to become piano experts need deliberate keyboard practice. This is different than just practicing hard to impress a teacher. For the best possible lessons, deliberate practice:
*Is designed specifically to improve performance. A mentor, teacher, or other professional instructor should create assignments for the student that is focused toward a goal, not just fun or nice to listen to. Because a student's learning needs change as they develop, teachers must be interactively engaged and in tune to what that particular student needs instead of just assuming the next song in the lesson book is the best assignment for the student.
*Is repetitive. Professional musicians have trained their bodies to play their instruments more than they have learned the workings of music. Like typing skills or a great golf swing or shooting a free-throw, mastering an instrument is more a matter of muscle memory than gaining knowledge.
*Requires feedback. It has been said that we judge others words by their meaning, but we judge our words by our intentions. The same is true in performing music. As we play, our brains makes adjustments in notes and timing so that we actually hear more what we meant to play than what we actually played. Even when listening to a recording, knowing who performed the piece greatly affects the evaluation of the piece! Aspiring musicians need a brutally honest, outside, feedback loop to draw out the details that need to be perfected.
*Is intellectually demanding. Of course, the theory portion of the lessons will be more intellectual, but there is always something else out there to think about once you perfect the detail you're working on now. The whole of becoming a musician has been summarized this way: "You're constantly trying to make a habit out of what you're thinking about now, so you have the brain power to think about the next thing." For more on this concept, read here.
*Isn't much fun. Everyone loves to perform well. But practicing isn't performing; it's the opposite of performing. In a performance, you try to minimize mistakes and draw out strengths. In practice, you focus on the mistakes and minimize the parts that are easy. Beginners and children should probably have more fun assignments, but for adults and advanced students, the fun in practice is the challenge and the promise of success in future performance, not the sound coming from the instrument!
For more details about deliberate practice from a researcher's perspective, please see the book "Outliers" by Malcolm Gladwell.