I'll walk you through on how you can add 3rd party apps to your QuickBooks Online (QBO) company.
Be advised, the Apps tab doesn't have an internal function to search for apps. You'll have to visit the QuickBooks Online Apps page. Ensure you're logged in as the Master admin or a Company admin, and from there, you can choose from the list of available apps that would work best for you.
If you need to either remove apps from your QBO company or transfer its ownership, you can follow the steps listed in this article: Transfer app ownership or disconnect apps. There are 2 sets of steps available, 1 for QBO and another for QBO Accountant.
Have other questions in mind? Post them below, and I'll get back to you.