Drupal is an open source project, built by the people who use it.
Patching Drupal Core
Check out the Drupal git repo to create patches from the right branch.
Patching contrib modules
Clone an issue branch repository within your project to work on a module in place.
Running tests
When you create a merge request, the automated tests will run.
Failing tests will show a warning on the page and prevent merging.
Run tests locally before pushing your work to catch issues yourself.
Use Drupal Contributions for Lando for a quick local testing environment.