Do I need to restart/stop/start IIS after Web.config updates?

Answer: No

Reason: Changes to the web.config will trigger the app to be reloaded by IIS as soon as there are 0 connections left to the app.

You can also stop and restart the app pool that the app is assigned to in order to make this happen. Recycling the app pool is not recommended and you should stop/start the respective app pool where you update. You do not need to stop and restart IIS itself just only the APP pool. 

Do I need to restart/stop/start IIS after DLL change/update?

Answer: No

Another query is when you change/update DLLs/drop new DLLs in the bin directory of your web application, it doesn't reset the IIS.

Reason: IIS monitors the /bin directory of your application.  Whenever a change is detected in DLLs, it will recycle the app and reload the new DLLs. So in case, you may want to confirm,  you should stop/start the respective app pool.