The tutorial explores the process of creating and fetching invoices using the QuickBooks Online API in PHP, emphasizing the use of OAuth2.0 for authentication. It guides developers through creating an Intuit Developer account, setting up an app, obtaining client keys, and defining a redirect URI to handle API responses. The tutorial includes creating a PHP project structure with multiple files, such as connectCompany.php for OAuth2.0 login, createInvoice.php for generating invoices, and getInvoices.php for retrieving invoice data. Additionally, it highlights the challenges of implementing such integrations and presents Merge as a unified API solution that simplifies connecting to multiple accounting platforms like QuickBooks, Netsuite, and Xero, thus saving development time and facilitating the expansion of applications with less effort.