Is parallel execution enabled or disabled by default in Oracle Standard Edition database version 12.1?
You say that VLDB and Partitioning Guide, Release 12.2


  About Initializing and Tuning Parameters for Parallel Execution
  
  Oracle Database computes defaults for the parallel execution parameters based on the
  value at database startup of CPU_COUNT and PARALLEL_THREADS_PER_CPU. 


Parameters are described in the Database Reference.

But a lot of features are not possible in the Standard Edition. Either they cannot be enabled or are not allowed. Some of the features of Enterprise Edition need extra license if you use them.


Most of the parallel features need Enterprise Edition but no additional license
Partitioning needs Enterprise Editition and and additional license


More about which edition has which feature can be found in the Licensing Information User Manual