There are many languages/APIs that one can use to build AEM projects.
AEM has a Rich Java API:
"The CQ5 Quickstart and Web Application."
You can use the AEM Java API to build OSGi bundles that can be used with services. For example, you can use the PageManager API to work with AEM pages.
See this as an example -- Adobe Experience Manager Help | Creating an Experience Manager 6.3 Page using the Page Manager API
AEM also supported Granite to help build UI such as component dialogs -- https://helpx.adobe.com/experience-manager/6-2/sites/developing/using/reference-materials.html
Reference Materials
To learn how to work with these Granite Resource types to - for example build component dialogs - see: Building Experience Manager Components using Granite/Coral Resource Types .
And you are correct in that you can use Sling APIs to work with AEM. You can write advanced use cases - like creating a custom AEM link rewritter service using Sling APIs -- Creating a Link Rewriter Service for Adobe Experience Manager 6.3
As far as JS frameworks - like JQuery. you can use them by using Clientlibs. See: . Adobe Experience Manager Help | Integrating the JQuery Framework into Adobe Experience Manager (an older article - but shows how to use JQuery and AEM)
The recommended way to build AEM components is to use HTL -- see this artilce to learn how to build an HTL component -- Scott's Digital Community: Creating an AEM HTML Template Language component that uses the WCMUsePojo...
Here is the HTL Spec-- htl-spec/SPECIFICATION.md at master · Adobe-Marketing-Cloud/htl-spec · GitHub
If you want to learn to work with HTL - we have webinars like this: Adobe AEM ask the experts - Sightly - YouTube
You can build components using HTL that show data like digital assets ,etc - see Scott's Digital Community: Creating a HTL Repeating Data Set 6.3 Component that uses Sling Models .
You can use the JCR API to work with AEM JCR nodes - see Adobe Experience Manager Help | Programmatically Accessing Adobe Experience Manager Content using th... .
You can even use the JCR API to write Java apps that can interact with the AEM JCR -- Scott's Digital Community: Developing a Java Swing Application that displays Adobe Experience Manage...
I hope this addressed your question. As you can see, there is a wide range of APIs you can use when working with AEM.
