"Be at home in" can be replaced with several synonyms such as feel comfortable in, be familiar with, be acquainted with, be conversant with, be knowledgeable about, be proficient in, be skilled in, be an expert in, be a master of, and be immersed in. Each of these synonyms conveys a similar meaning to "be at home in" which is to be well-versed or accomplished in a particular subject, activity, or environment. Using these synonyms, one can convey a deeper understanding of a topic or skillset, and showcase their expertise in a particular field.