Some companies will use a contract manufacturer to produce their cars for them. For example Fisker (about bankrupt) used Magna Stehr in austria to produce their EVs.

When you talk about a base platform I assume you are meaning an entire drive train? Most pieces of a car are going to be produced elsewhere and then assembled. Canoo isn’t making their own batteries or motors. Those come from battery and motor manufacturers. Canoo or a similar company might even buy structural battery pack built to spec by CATL.