extendは2次元(線)でのばす、expandは3次元(空間)でひろげるイメージ

extend

[transitive] extend something to make something longer or larger

・ to extend a fence/road

・There are plans to extend the subway line.

Oxford Learner’s Dictionary “extend”

expand

[intransitive, transitive] to become greater in size, number, or importance; to make something greater in size, number, or importance

Metals expand when they are heated.

Student numbers are expanding rapidly.

A child’s vocabulary expands through reading. The waist expands to fit all sizes.

Oxford Learner’s Dictionary “expand”