I used to use the meteo framework before, and I think if it's similar. By the way, is it possible to use MongoDB on the server side and Triplit on the React side?
Or do I have to use a new database called Triplit?"
It's not currently possible to use MongoDB with Triplit, you would use Triplit's server instead but it's very easy to setup with React: https://www.triplit.dev/docs/quick-start