Interface Relationship Diagram?

What tool do you go to, to map out interfaces and their relationships. For example, we have a public API contract that is powered by our database and an external API source like Stripe's subscription object. I'd like to map out how these interfaces are linked.

In the past, I've utilized ERD tools but found them to be clunky and expect that your designing a database. Maybe I just have used the right one but please share if you have a go to tool!