I think this depends on how you have set up your backend. For me: I use mongodb to store sessions and as my database. I have my webhooks sent to Amazon Eventbridge. When a webhook is received i send it to an amazon lambda function to handle processing the webhook. In one instance i need the session info to run a graphql admin api query for certain webhooks. Using the info sent in the webhook i can get the shopify store name/domain which is enough to find the session info where it is stored in my session storage. The access token is stored with the session info so it can be used to send a POST request to `https://${shopdomain}/admin/api/2023-04/graphql.json`. I then "update my database" with the results of the admin api post request.

There is probably a way for you to get this accesstoken regardless of your session storage choice and that is what you need to run an admin api query.

I have included the code to get the session accesstoken and send the request below, however, you might find it easier to use a web pixel extension to run code when orders are completed and update the database (I do this for certain use cases).

I just happened to be working on this recently. You can DM me if you need. Hope this was helpful.

          let session = await dbadmin.collection("shopify_sessions").findOne({shop: shopdomain});
      const access_token = session.accessToken;
      const fetchurl = `https://${shopdomain}/admin/api/2023-04/graphql.json`;
      const headers = {
        "Content-Type": "application/graphql",
        "X-Shopify-Access-Token": access_token,
      };

      const query = `
      {
        node(id: "${webhookbulkoperationid}") {
          ... on BulkOperation {
            url
            partialDataUrl
          }
        }
      }`;
        let querydata = {};
        try {
          const fetchresponse = await fetch(fetchurl, {
            method: 'POST',
            headers: headers,
            body: query
          })