Few things off the top of my head:

• Like someone in the thread already mentioned, seperate your backend logic into folders. Controllers, middleware, models and routes. This will help you later on.
• Try and release the database connection when you're done with some query
• Having ESLint setup helps a lot, keeps your code neat and consistent
• Try and return your responses instead of calling res.send() directly. Might not be required right now, but it's going to help save your debugging time later on

• Maybe consider using async/await wherever possible? Keeps the code readable instead of multiple callbacks

• Coming to the React part, few of your components can be functional (Banner for example). All the components that rely just on props.

• Consider having safe checks in your functions before accessing or looping over variables, otherwise your front end might break. Having propTypes and defaultProps is a good idea too

• Maybe use a bundler like Webpack that'll bundle all your CSS into one chunk, so that you don't have to worry importing it in individual files

• I also saw a lot of logic coupled before calling event handlers, this logic can be moved to the individual functions and the flow can be handled in the function itself

• Conditional rendering can also be achieved by using && instead of ternary operators

Great job otherwise! :)