Queries running parallel with default maxdop
The simple answer is that 64 is the DOP limit when zero is specified:


  To enable the server to determine the maximum degree of parallelism,
  set this option to 0, the default value. Setting maximum degree of
  parallelism to 0 allows SQL Server to use all the available processors
  up to 64 processors.


A single query can use more than 64 cores if you set that specific number (i.e. not zero):


  Set the value to a number from 1 to 32,767 to specify the maximum number of processor cores that can be used by a single query execution


See also SQL Server MAX DOP Beyond 64 – Is That Possible?

As for how SQL Server will allocate threads, this blog post is the closest I've seen to official documentation:

SQL Server Parallel Query Placement Decision Logic