How do relations work in NOSQL?
In your question you are using "SQL" and "relational" to mean the same thing.  In the same way you can think of "NoSQL" as meaning "not relational" i.e. there is no primary key / foreign key concept; there is no "normalisation" enforced or implied; there is no real meaning of the words "table" or "column".  To a key-value store all it knows about is a big collection of bytes you pass to it - the "value" - and a smaller collection of bytes - the "key" - used to index that big collection.  What is inside that big collection and how it maps onto byte in other values in the database is entirely down to your application and the quality of the code in it.

Other NoSQL DBMSs, however, do allow you to define what is inside the "value".  They hold a schema for the data, much like relational databases do.  Cassandra is an example of this.

So it is up to you and how you implement your application.  You could set up your key-value store to mimic a relational data model with surrogate ids.  This would require a lot of reads and lose the point of a NoSQL solution.  You could set up your key-values to store all denormalised values at the instant the record was written.  This will be fast for retrieval but slow for updates.  You have to choose what's best for your system and implement it accordingly.  NoSQL is not magic.  You still have a lot of design and development to do.

The term NoSQL is very broad.  It has more to do with marketing than with a technology.  Still less to do with maths such as the set theory underpinning relational databases.  Different NoSQL products work differently.  You will not know all of "NoSQL" by reading just Parse.com.

Understand your problem.  Understand the compromises you are prepared to make to achieve your performance or consistency goals and choose the product which best matches your requirement.