Spark: How to know how spark is doing partitioning based on which key?

You have to distinguish between two different things:

partitioning as distributing data between partitions depending on a value of the key which is limited only to the PairwiseRDDs (RDD[(T, U)]). This creates a relationship between partition and the set of keys which can be found on a given partition.
partitioning as splitting input into multiple partitions where data is simply divided into chunks containing consecutive records to enable distributed computation. Exact logic depends on a specific source but it is either number of records or size of a chunk.

In case of parallelize data is evenly distributed between partitions using indices. In case of HadoopInputFormats (like textFile) it depends on properties like mapreduce.input.fileinputformat.split.minsize / mapreduce.input.fileinputformat.split.maxsize.
So default partitioning scheme is simply none because partitioning is not applicable to all RDDs. For operations which require partitioning on a PairwiseRDD (aggregateByKey, reduceByKey etc.) default method is use hash partitioning.

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