The PR needs to have someone who knows the whole thing.

Having several people review each separate parts but not understanding the others' can cause interaction bugs. If such bugs cannot happen (say, due to modularity, or type safety guarantees etc), then it won't be the case where you need to have a partial approve.

I am not a fan of partial approve. Either you think the code is approvable, or it isn't.

Domains of expertise are a thing. E.g. Google had "readability" which was the code style and opinioned language expertise that one person might have even without the deep system knowledge for a PR.

You can require approvals from N domains from (potentially) different people.