core js codes

in core/server/services/themes/active.js:

        // Set the views and engine
        siteApp.set('views', this.path);
        siteApp.engine('hbs', engine.configure(this.partialsPath));

Refers

https://expressjs.com/en/guide/using-template-engines.html
https://docs.ghost.org/api/handlebars-themes/