At our company, technical design reviews happen before any work on a feature begins. I thought it's the case everywhere. Why would anyone review a design/ architecture after it's already been built?
It's also never just an approval here, most devs usually have something to say, and a technical design can be completely rewritten/rethought.
Also we don't formalize it too much, it's just a few folks in an informal setting discussing diagrams for 1-2 hours. In the end we decide if it looks good enough after all the tweaks, or we need another session.