How to run custom Shopify app for multiple merchants

I am creating a custom app(s) that should run for multiple merchants.

I want to run a central server for all my custom apps.

Shopify has 2 app types:

• Custom App: Limited to installation on one merchant store
• Public App: Unlimited merchant installs but needs to be reviewed by Shopify to protect trust and security.

The are several reasons we do not make this app public:

• To avoid implementing the Shopify Billing API.
• To avoid waiting for the app listing review.
• The is no need for this App to be public on the Appstore

I understand that I need to create a single Shopify Custom App for each merchant, but how do I handle it on the server-side?

Is there any way to make the createShopifyAuth middleware to establish auth dynamically?

server/server.js

const { SHOPIFY_API_SECRET, SHOPIFY_API_KEY, SCOPES } = process.env;
server.use(
// TODO: Retrive shopID
// TODO: Fetch API_KEY, and API_SECRET from the db by given shopID
    createShopifyAuth({
      apiKey: SHOPIFY_API_KEY, // TODO: Use the API_KEY for given store
      secret: SHOPIFY_API_SECRET, // TODO: Use the API_SECRET for given store
      scopes: [SCOPES],

      async afterAuth(ctx) {
        //Auth token and shop available in session
        //Redirect to shop upon auth
        const { shop, accessToken } = ctx.session;
        await handlers.registerWebhooks(
          shop,
          accessToken,
          "ORDERS_PAID",
          "/webhooks/orders/paid",
          ApiVersion.October19
        );
        await handlers.registerWebhooks(
          shop,
          accessToken,
          "ORDERS_FULFILLED",
          "/webhooks/orders/fulfilled",
          ApiVersion.October19
        );
        ctx.cookies.set("shopOrigin", shop, {
          httpOnly: false,
          secure: true,
          sameSite: "none",
        });
        server.context.client = await handlers.createClient(shop, accessToken);

        await handlers.getOneTimeUrl(ctx);
        ctx.redirect("/");
      },
    })