In the function extractParams athttps://github.com/episerver/content-graph-js-sdk/blob/main/samples/musicfestival-frontend-react/src/helpers/urlHelper.ts#L20, contentId and workId is read from the payload of of the previewToken :
const contentId = payload.c_id && parseInt(payload.c_id!.toString());

But the documentation athttps://docs.developers.optimizely.com/platform-optimizely/v1.4.0-optimizely-graph/docs/on-page-editing-using-content-graph#setup-decoupled-site states that we should not depend on the content of the token :

"Note

The preview token is only meant to be used to query data from Optimizely Graph. Do not depend on the contents of the token itself as they are subject to change in the future.
"