Picture the auto and homeowners policies as the ground floor of a building. Excess and umbrella policies add floors on top. An excess policy is a taller copy of the floor below. An umbrella is taller and also a little wider: it can cover some rooms the ground floor leaves out.
Excess liability (true or follow-form excess): only adds limits. It follows the underlying policy's terms, so if the underlying policy does not cover a claim, the excess policy does not cover it either.
Umbrella liability: adds limits above the underlying policies and can also respond to some claims the underlying policies exclude, usually after the insured pays a self-insured retention.
California's Insurance Code defines the commercial versions in §676.6(b): an umbrella policy is written over one or more underlying liability policies or over a specified amount of self-insured retention. An excess liability policy is written over underlying liability policies, and that definition expressly includes policies written over umbrellas.
Layers pay from the bottom up. Example: a $1,000,000 CGL, a $3,000,000 umbrella above it, and a $5,000,000 excess policy above the umbrella. A covered $6,000,000 judgment is paid $1,000,000 by the CGL, $3,000,000 by the umbrella and $2,000,000 by the excess policy. The layers do not split the loss in proportion.