An example of what an extension library using @rbxts/expect would look like.
The library directory contains a project that publishes the extension @rbxts/expect-rust-types.
The consumer directory contains a project that uses the extension @rbxts/expect-rust-types.
Both of these libraries should help provide you with the context you need to understand how to create an extension library, and how people will use that extension library.
To learn more about extension libraries, and how they work, take a look at the Extension Libraries Guide.