Couple of blogs to start with

https://unlocklearning.in/dynamic-dropdown-in-aem/

http://www.sgaemsolutions.com/2019/01/dynamically-populate-drop-down-values.html 

Hello @naresh536 ,

I created a similar type of dialog generation last year. My problem was like this:
I have a component, Component dropdown fields are dynamic. Based on the selection fields checkboxes were shown and hidden which was also dynamic. And all those data came from a service REST API.

My Solution:

I create a dialog like this, where the selected items and checkbox was not present.

At the time of dialog loading, I created the rest of the fields by using Java under /apps/myproj/component/mycomponet/cq:dialog/...
For this, I created a service user and give that user READ, WRITE, MODIFY, DELETE permission. (/apps/myproj/component/mycomponet)

And the Java code is something like this. Hope you understand.

Cons:
Since we write under /apps, in the parallel time of authoring value will be changed by other authors.
In my case, this component was not heavily used and need to configuration sometimes. That's why the overlapping change was very rare.

You can check out to get reference how to create granite/Coral field dynamically.

https://aemlab.blogspot.com/2022/01/aemaacs-touch-ui-dialog-dynamic-dropdown.html 

Hello @naresh536 ,
Since you don't want to create a node under /apps there are another solutions.

Solution 1:

You can use Datasource.

Here is a sample code, where you will call an HTML file and on that HTML you will call a Java Model class. That model will call the API and create a json for you.

XML:

<myList
        jcr:primaryType="nt:unstructured"
        sling:resourceType="granite/ui/components/coral/foundation/form/select"
        fieldLabel="Select An Item"
        name="./selectItem">
    <datasource
            jcr:primaryType="nt:unstructured"
            sling:resourceType="/apps/my-proj/datasource/speciality/speciality.html"/>
</myList>

HTML:

<sly data-sly-use.data="com.myProj.modal.MyModel"></sly>

Java:

List<Resource> resourceList = new ArrayList<>();
ValueMap vm = new ValueMapDecorator(new HashMap<>());
vm.put("value",value);
vm.put("text",text);
resourceList.add(new ValueMapResource(resolver, new ResourceMetadata(), JcrConstants.NT_UNSTRUCTURED, vm));

DataSource ds = new SimpleDataSource(resourceList.iterator());
this.getRequest().setAttribute(DataSource.class.getName(), ds);

Solution 2:

You call Java Servlet directly from the dialog XML

XML:

<myList
        jcr:primaryType="nt:unstructured"
        sling:resourceType="granite/ui/components/coral/foundation/form/select"
        fieldLabel="Select An Item"
        name="./selectItem">
    <datasource
            jcr:primaryType="nt:unstructured"
            sling:resourceType="your/servlet/path"
            parameter="optional your value"/>
</myList>

Java:

Same as the previous one.

Reference Link: https://levelup.gitconnected.com/aem-populate-a-drop-down-with-a-data-source-edafada90cc4