The monthly Google System Release Notes primarily detail what’s new in Play services, Play Store, and Play system update across Android phones/tablets, Wear OS, Google/Android TV, Auto, and PC.
Some features apply to end users, while others are aimed at developers.
The following first-party apps list produced by 9to5Google comprise the “Google System”:
Adaptive Connectivity Services: Search the “All apps” list in Settings and open App info for the version number
Android System Intelligence: App info
Android WebView: App info
Device Health Services: App info
Settings Services: App info
Google Play Store: App info
Google Play system update: Settings > About phone > Android version
SIM Manager: App info
N.B. Just because a feature appears in the Google changelog does not mean it is widely available yet - some new capabilities can take months to fully launch.
The internet hosts many sites that will give you free advice and support for your particular smartphone as well as lots of general information on new products as they come along.
Manuals & Guides
A good place to start is your smartphone manufacturer's website - try googling the name of your smartphone to obtain details of popular brands such as Hisense, or HTC, or LG, or Meizu, or Nexus, or Samsung, or Sony, or Toshiba, or Xiaomi etc.
Forums
There are some very useful specialist sites like androidforums that will help you to solve all kinds of problems.
Reference Sites
This site provides facilities for finding out all kinds of news, updates, and fixes for the common issues affecting Android Apps and smartphones.
Examples of some articles available @ AndroidPIT
7 tips to make you an Android expert - 8th July 2015
10 Best Apps of 2015 - 10th June 2015
Another great site that is well worth checking out.
These guys have put together a really good introductory tutorial to help you get to grips with your Android phone's features.
Most of these sites let you enrol / subscribe free of charge so that you can receive regular emails on different topics of interest. All you need to do is supply an email address, select any items that interest you, then sit back and wait to receive all the latest news. If you start to get too much stuff from them you can always go back and reduce or cancel your subscriptions.