App.js

// #1
// can pass by name
<Button children={el2} />
const el = React.createElement(Button, { children: el2 })
  
// or by position
<Button>{el2}</Button>
const el = React.createElement(Button, null, el2)
// React theoretically turns it into this once you're in the component:
const el = React.createElement(Button, { children: el2 })

// #2 it flattens multiple children into an array
  <Button>
    <span/>
    {stuff.map(() => <span/>)}
    <span/>
  </Button>

const el = React.createElement(Button, null, el2, [el3, el4], el5)

const el = React.createElement(Button, { children: [el2, el3, el4, el5] })