These are stock expressions. "familiar territory" means a situation, subject, location, etc. that you are familiar with, or that is generally familiar to people.
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soheil1'Stomping ground' has this meaning, right?"stomping ground" is used somewhat differently in sentences; usually it is preceded by "my", "his", "her", etc., and it always refers to a geographical location or region that a person frequents, whereas "familiar territory" often has an abstract meaning.