Deployments can be triggered through GitLab's CI/CD. Currently .gitlab-ci.yml has a deploy_site stage which is configured to deploy to Heroku on pushes and merge requests to either master or staging.
Deployments can also be done through the terminal with Git and the heroku CLI, however best practice is to go through the GitLab CI/CD.
For information on hosting such as Heroku app names see hosting documentation.
After the Deploy to production stage of the development life cycle wait for the pipeline to pass, click on the Manual job dropdown and then click deploy_production. This will require maintainer permission for the project on GitLab.
There is an additional manual task deploy_content_prep. This should generally be run whenever a production deployment is preformed, as it keeps the codebase and database on the Content Prep server in sync with production.
Unlike production, deployments to staging are done automatically through GitLab CI/CD, simply push to staging.