The Pipelines run in the Azure DevOps infrastructure project. They are defined in YAML in the pipelines file.
There is a CI pipeline which runs validation and various checks. This is linked to Pull Requests, so it must pass before it is possible to merge.
The CD pipeline deploys to the various environments. This is triggered automatically to the Dev environment when there is a merge to the main branch. For manual runs, you must choose the region and stack you wish to deploy.