The phrase "does one thing" can be replaced with multiple synonyms depending on the context. For instance, it can be substituted with "specializes in," "focuses on," "concentrates on," "devotes to," or "centers around." These synonyms capture the essence of doing one thing and doing it well. Alternatively, one can use phrases such as "the main objective of," "the primary goal of," or "the core purpose of" to convey the meaning of doing one thing exclusively. Ultimately, the appropriate substitute depends on the context and desired tone of the sentence, but these synonyms offer a range of options for writers to convey their ideas effectively.