the video gives a detail procedural on the process, this is going to be a quick breakdown of the different nodes used in the node tree.

step 1 Volume Cube:

the volume cube nodes gives you a volume in a shape of a cube that we later turn into a sphere, there is a mesh to volume node that you can use to turn a sphere mesh into a volume sphere directly but the reason we are not using it, is because its density input does not support fields only supports a float value, the only node that could do what we want easily is the volume cube as its density input supports a noise texture.

Step 2: turn volume cube to volume sphere:

to do this we are going to use the 3 nodes in the image above inside box 1

if you connect a gradient texture with a type set to sphere directly into the volume cube node, you will get a spherical volume, the position and vector math node set to scale is there to help control the scale of the sphere.

step 3: add dents into the spere to make continents and oceans:

the nodes in box 3 in the above image are used for generating the volume and then the volume to mesh node converts the results of the volume cube to a mesh, right now you should see a sphere mesh when you view the outputs of the volume to mesh node.

to add oceans and continents we are going to subtract a noise texture from the sphere gradient we have using a math node set to subtract.

the color ramp on the noise is used to control the contrast of the noise

step: 4 adding the ocean:

the ocean is a separate sphere all together created from a box mesh with a subdivision surface which gives use better polygon distribution than a uv sphere later when we are creating the ocean material

to complete the ocean, we need an ocean shader, for this we lets first join the ocean to the continents use a join geometry then add a proximity nodes using the continents sphere as the target, then we can save the proximity mask in a named variable called ocean depth. you can now create a material for the ocean and add a set material node to the ocean sphere so that it uses this material.

download project file here