I am a beginner SharePoint developer, here is my question
The people picker fields in SharePoint, fetches data from user profile or AD?
How do I sync user profile to AD?
Answering your questions specifically:
By default SharePoint doesn't get a lot of profile information for the AD and only when a use first accesses the SharePoint Farm.
If you want to have full profile information, you need to configure the User Profile service. And this is not always en easy thing to do. Depending on your SharePoint version, I would opt for AD import if you need only info from the AD. If you need to write back data to the AD you need to configure a sync.
Your options all depend on the version you are running and can be very different between 2010, 2013 and 2016.
This gives you an idea how to properly configure the sync for 2010 (and I think most of it applies to 2013 as well) http://www.harbar.net/articles/sp2010ups.aspx
And for the AD import: http://www.harbar.net/archive/2012/07/23/sp13adi.aspx
I'm only referencing this site http://www.harbar.net, as he is the guru regarding User Profiles.