When a user leaves your organization, your IT admin will typically suspend or delete the user's account in your identity provider dashboard. As part of that workflow, the admin can usually choose to transfer data and permissions for the departing user to another user in your organization; this includes permission for access to Coda docs.
If your organization is on an Enterprise plan, then one of your Organization Admins can change doc ownership from the Organization Settings panel.
If you are not on an Enterprise plan, you'll need to contact Coda support for assistance with this. You can read more about the steps required under the "Non-Enterprise" section of this article.
Finally, if your organization is connected to Google Drive, your GSuite Admin can handle this on your behalf. See this Google help article for more information: https://support.google.com/a/answer/1247799. Note that very few workspaces are now connected to GDrive.