What is the relationship between maxExecutors, num-executors and initialExecutors when using Spark on YARN?

At first, I read this article, which says spark.dynamicAllocation.maxExecutors will have value equal to num-executors if spark.dynamicAllocation.maxExecutors is not explicitly set. However, from the following part of this article, it says "--num-executors or spark.executor.instances acts as a minimum number of executors with a default value of 2", And it confuses me.

1. My first question is what is the usage of --num-executors in Spark 2.x or later versions? Does it act like a obsolete option which is useful before dynamicAllocation? When dynamicAllocation is introduced, --num-executors and --max-executors act more like some default values to spark.dynamicAllocation.*?

2. What is the difference between --conf spark.dynamicAllocation.maxExecutors and --max-executor? Does the later acts like an alias to the former?

Meanwhile, the article does not mention the relationship between num-executors and spark.dynamicAllocation.initialExecutors. So I make a experiment in which I send the following arguments:

--conf spark.dynamicAllocation.minExecutors=2  --conf spark.dynamicAllocation.maxExecutors=20  --conf spark.dynamicAllocation.enabled=true --conf spark.dynamicAllocation.initialExecutors=3 --conf spark.dynamicAllocation.maxExecutors=10  --num-executors 0  --driver-memory 1g  --executor-memory 1g  --executor-cores 2  

And it turns out that initially 3 executor is allocated(correspond to initialExecutors) and then reduced to 2(correspond to minExecutors), and it seems that --num-executors is useless here. However, the article says "-num-executors or spark.executor.instances acts as a minimum number of executors", so there is a contradiction now.

3. My third question is what is the relationship between num-executors and spark.dynamicAllocation.initialExecutors, is spark.dynamicAllocation.initialExecutors prior to num-executors? From the document, I find when num-executors is set larger than spark.dynamicAllocation.initialExecutors, it will override spark.dynamicAllocation.initialExecutors.