Sashkos answer below is pretty good.

To get a good understanding of the difference between Apollo and Relay I would suggest going through both https://learnrelay.org/ and https://www.learnapollo.com/ - should be doable in a couple hours.

After that you could take a look at the Graphcool Console https://github.com/graphcool/console It's the most complex open source app build with Relay that I'm aware of.