How to use the sphere generator
Pick Sphere, Dome, Ellipsoid or Cylinder and set the size. Choose Solid or Hollow; hollow shapes let you set how many blocks thick the shell is, and hollow cylinders can have open or closed ends.
Move between layers with the slider, the arrow buttons, or Page Up and Page Down. Layer 1 is the bottom. Tap Layer done as you finish each one: the tool moves you up a layer and remembers your progress in this browser.
Drag the 3D preview to turn it. The current layer glows amber, the layers below are solid and the layers above are see-through, so you always know where you are in the build.
How each layer is worked out
A block is part of the shape when its center sits inside the true sphere or ellipsoid. Every flat slice of a sphere is a circle, so each layer here matches the circle generator for the same slice.
A hollow shell keeps every block with a face open to the outside, plus enough blocks to make the wall as thick as you asked. That makes the shell sealed: nothing can pass straight through it, and it has no holes from any side.
Building tips
Mark the middle column first and build every layer around it. The amber outline shows the middle block on odd sizes; on even sizes the middle is the corner where four blocks meet.
Start with the widest layer if you want to check the space it needs, then build up and down from there. For the top layers of a big sphere, scaffolding is the easiest way to reach.
Odd sizes are easier to center on a single block, like a beacon or a door in a dome.