[be] {one's} department

to be responsible for something, showing a sense of authority in that area

Isn't handling customer inquiries your department, Chris?
Coordinating project timelines is Alex's department in the project management team.
Customer service is my department, so feel free to reach out if you have any issues.
Strategic planning is Laura's department, and she plays a pivotal role in shaping the company's long-term vision and goals.
Safety protocols are the supervisor's department; they ensure a secure work environment.

to know a lot about something, indicating a sense of expertise in the matter

I can look into it, but legal matters are not exactly my department.
I'll do my best, but graphic design isn't really my department.
Jane, being a seasoned biologist, considers marine life to be her department; she can identify various species effortlessly.
When it comes to computer programming, it's definitely Mark's department; he's our go-to expert for coding challenges.
As an experienced chef, culinary arts are Chef Rodriguez's department; he has mastered the art of creating exquisite dishes.