Not sure about HelloFresh but at Zalando there are rules for that all fragments are supposed to follow so that this doesn't happen. In practice that means everyone has to use React in the same version, there is a shared library of components for grids, buttons to achieve consistency in look and feel and so on.

You could have multiple SPAs in the same page but you don't actually do that in practice.

You do this not because this is simple, not to create a better user experience but so that you can have 100+ teams working on the site without them being stuck because every change needs to be carefully coordinated with everyone else.