This would be good to go through : https://github.com/adobe/aem-guides-wknd https://docs.adobe.com/content/help/en/experience-manager-learn/getting-started-wknd-tutorial-develop/overview.html

And the study guide is here : https://learning.adobe.com/content/dam/adls/certification/samples/en/AD0-E105%20Adobe%20Experience%20Manager%20Lead%20Developer_EN.pdf

That exam code is no longer offered -- the updated exam is : AD0-E105 Adobe Experience Manager Sites Lead Developer  

Yes it should work. What tends to happen though is that these 3rd party libraries are using some version of ECMASCRIPT that is newer than what GCC supports. So the minification fails for a specific library. The GCC processor will log something like :  ERROR - This language feature is only supported ...

Maybe I'm not understanding your ask but you can freely customize the side rail on various editor pages here :  <aem>:<port>/libs/cq/core/content/tools/customsearch/searchfacetformlister.html

This past answer from Paul describes this well :  https://experienceleaguecommunities.adobe.com/t5/Adobe-Experience-Manager/Language-Copy-vs-Live-Copy/qaq-p/189139/comment-id/14611 It removes the LiveRelationship so that no Rollouts impact the Language Copy.Typically LanguageCopies are used within a...

https://docs.adobe.com/content/docs/en/spec/jsr170/javadocs/jcr-2.0/javax/jcr/RangeIterator.html#getSize() Returns the total number of of items available through this iterator. For example, for some node N, N.getNodes().getSize() returns the number of child nodes of N visible through the current Ses...

I think you're hitting this : https://github.com/adobe/aem-core-wcm-components/issues/902or this : https://github.com/adobe/aem-core-wcm-components/issues/881  

When trying to understand a thread dump the key is to look at the RUNNABLE threads to understand why they hold contention over the WAITING threads.   Also CQ 5.6 has been end of life since 2016.   Upgrade

Reviewed : at com.adobe.granite.workflow.core.util.SecurityUtil.getAuthorizable(SecurityUtil.java:49) [com.adobe.granite.workflow.core:2.0.168.CQ640-B0006] at com.adobe.granite.workflow.core.jcr.WorkItemManager.getBackRoutes(WorkItemManager.java:806) [com.adobe.granite.workflow.core:2.0.168.CQ640-B0...