In marketing-speak, "Build/Design your own..." usually means "Choose from a series of pre-configured options," as in Build Your Own Sundae or Design Your Own Sneakers. So it goes with a company called Trexa, except their material isn't ice cream or shoe leather; it's electric car parts.
Trexa's lithium-powered, all-wheel-drive vehicle development platform is intended to "[enable] specialty vehicle developers to design and build custom 'vehicle apps'" by dialing in the terrain type and ride height, and scaling the wheelbase. Conceivably this means you could build everything from an off-road Smart Car (not that you'd want to) to a stretch limousine. Trexa's battery systems are also scalable and modular; presumably you'd simply add more for more power. They're even working on docking systems for interchangeable bodies and remote=controlled vehicles.