Your account runs the apps. Syntica runs the workflow.
Each environment gets its own server in your account, behind a load balancer in your account, with HTTPS certificates issued there. Syntica keeps what it needs to run the workflow: the chat connection, deploys, who can do what, the activity log, app logs, settings that are not secret, and the organisation-wide secrets and connection credentials, encrypted.
On hosted Syntica, the same apps run on their own servers, one per environment, in our AWS account in Frankfurt. You can mix both in one organisation.