A system can have closed components and still be open, and many open components and still be closed.

The question is, can the user and the ecosystem route around any one component if they want to? Do users have meaningful agency on how to combine the components?

If the way the user interacts with the whole system is mediated by one single provider at the top (often an aggregator) then they can't escape the value judgements and decisions of the provider; the system is closed, even if many components of it are "open".

But imagine a system where the user is empowered to mix and match and stitch together many components in any way they see fit.

Even if many of the components they're stitching together are individually closed, the system is still meaningfully open.

The openness of a system is primarily determined by the configurability and combinability of the layer closest to the user.