Freeform with project.yaml? On my development instance, I added a freeform form. Now I would like to deploy that form to the staging, or later, live site. 

When committing my changes, I noticed that nothing was added to project.yaml. So I guess the changes are just saved in the DB and will not be synced at git-based deploy.

Is that by design? Can I enable useProjectConfigFile somewhere?

If not: how do I export the form definitions and deploy them to another instance? By MySQL?
Project config is designed to sync the architecture of your site, not the content, so structural changes can be made without affecting the content. Forms are seen as content, like entries, so this would be the expected behaviour.
The suggested workflow would be to create the form on the production site, then pull to local environment to do any front-end work, then push those changes to production.
The nystudio107 craft scripts are pretty useful for syncing databases and assets if needed.