I’m generalising slightly, but if you think of it like this it might be a bit clearer:
React is great for building user interfaces (“front end”), so can be used to put together the bits of your web app that the end user gets to see,
- and SQLite is the data store that holds the goods.
To put it in the context of a small blog application, the web pages that your readers see would be made with React, the work involved in fetching all your blog entries from the month of June, 2019 would be processed by Express, and the actual content of your blog posts - the title, body, tags, comments etc - would be stored in tables in SQLite.
That help at all?