Build up and Start up

Build up verb - To become greater in extent, volume, amount, or number.

Start up and build up are semantically related In some cases you can use "Start up" instead a verb "Build up".

Start up verb - Get going or set in motion.

Build up and start up are semantically related in activate topic. Sometimes you can use "Build up" instead a verb "Start up", if it concerns topics such as energize.