I've been working through PRs since around 2011 and I don't think I've ever seen a place where PRs were intentionally used to handle design issues. Occasionally design problems will get brought up in PR, but that's always been considered a failure of earlier planning when it happens.

Depending on the company, the design discussion you're talking about has happened:

1. In verbal discussions with other devs before fingers ever touch keyboards

2. In long-form writing (Text docs, then Google docs, then later Notion)

3. In per-project slack channels

4. Out loud, on a whiteboard.

Right now it's very much #4 because my company is 2 devs and a founder. But even here, I wouldn't expect to get anything non-trivial into pull request before figuring out the overall structure with the other dev verbally or in writing.