Right now there isn't just one way to set up a custom theme in PWA studio, so i'm going to show the core files as of 3.0 for an answer to the question.
In the footer file add in this code:
src/pwa-studio/packages/venia-concept/src/components/Footer/footer.js
import CmsBlock from '../CmsBlock';
render() {
...
return (
<footer className={classes.root}>
...
<CmsBlock identifiers={'cms-block-name'} />
...
</footer>
)
}
This will pull in your cms by it's name into the react component. You can place the component anywhere inside the return statement. If you need to add styling to the component, i would recommend wrapping it and setting a className
on the wrapper.