After opening a PR, which stage begins testing? Is it as soon as the developer opens the PR, or does code review happen first?

Testing usually begins after the developer has opened a Pull Request and the code review has passed at least a basic level. This is because testers need to work with more stable and review-ready code. In some teams, automated tests are run immediately upon opening a PR, but full manual or in-depth testing begins after a successful code review to avoid testing low-quality or incomplete code.