How do libraries to provide users a bindData function?

I need to copy the behaviour of, for example, the “create” function of Axios:

// fileOne.js
import axios from 'axios';


const instance = axios.create({
  baseURL: 'https://some-domain.com/api'
});

Then to use that baseURL in another file, with another function like this:

// fileTwo.js
import axios from 'axios';

axios.get('/user/12345'); // https://some-domain.com/api/user/12345

Getting this as result:
https://some-domain.com/api/user/12345

How does Axios to bind the baseURL data in his library.

I’m looking that library but i don’t understand how they do that.