Platform - definition
A car platform is the foundation on which a vehicle is based. Its design and specifications determine the rigidity of the car, its suspension design, the interior space and the relevant engineering choices such as motor and transmission.
Some manufacturers modify existing ICE platforms, some create EV platforms from a clean sheet of paper, while some others adopt a hybrid approach to the matter using both methods on different vehicles.
You can find out about the drawbacks and benefits of each approach from this article.