Hi,
You can start looking into OOTB available form controls ftl files under /share/WEB-INF/classes/alfresco/site-webscrtipts/org/alfresco/components/form/controls.
After creating your custom ftl file, you can place it under "shared" location that is tomcat/shared/classes/alfresco/web-extension/site-webscrtipts/org/alfresco/components/form/comtrols.
And in your share-config-custom file, you can provide your custom ftl file path like,
- <control template="/org/alfresco/components/form/controls/custom-control.ftl">.
Hope this helps.