Phrases

The dictionary also includes definition of phrases. An example is: the definition of "command of something" defined in Longman Dictionary of Contemporary English (https://www.ldoceonline.com/dictionary/command-of-something) as
"knowledge of something, especially a language, or ability to use something."

How does one (a person) show command of a topic? Professionals write articles, speak on the topic at conferences, explain difficult concepts on the topic to others or use other ways to show that they are experts in a particular area/topic.