without having to add extra frames
you don't animate 3D models frame by frame, you use keyframes, which is like... you put the model where you want it to be at one point, make a keyframe. move it to the next point, another keyframe. etc. and then when it's time to play back the animation you just see it moving between those two keyframes
there's a lot more to it than just that I guess but that's basically how it goes. 2D animation is often like that these days too, when it's not hand-drawn
