What if you want someone to look at a portion of it but they don't know enough to approve the whole thing. They give +1

Someone else knows the other portion well and sees the +1 and decides to +2.

In practice this ends the stalemate where partial owners don't feel confident to approve the whole thing

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.

To be clear, that is an opinion, not an objective truth.

Some people think that PR status can also communicate rationales and partial approvals.

Some think that should be done with tags and comments.

Lots of request systems have multiple stages between "open" and "resolved".