The difference between a stage piano and a keyboard is mainly the way they are constructed and the purpose for which they are used.
Stage pianos tend to be compact and lightweight, and are commonly used by professional pianists and touring musicians. They typically offer high quality sound production and a realistic piano feel as they feature hammer action and weighted keys similar to playing an acoustic piano. Stage pianos tend to be more expensive than keyboards because they are designed for a specific application and have higher quality sound engines.
Keyboards, on the other hand, tend to be cheaper and more versatile than stage pianos. Commonly used by amateur musicians and beginners, they typically offer a variety of sounds and features, such as different instrument sounds, rhythms, and effects. Keyboards usually do not have hammer mechanics and weighted keys, which means that the playing feel is less realistic than that of a stage piano. They are also typically larger and heavier than stage pianos because they have more features and connections.
In summary, stage pianos are primarily designed for professional pianists and musicians looking for a compact, lightweight instrument with high-quality sound production and realistic playing feel. Keyboards, on the other hand, are more versatile and less expensive, and are good for beginners and hobbyists looking for a variety of sounds and features.