In my opinion on the front end these days in the vast majority of cases you'd be insane to use anything but flutter. If you compare to all the other alternatives you can develop better apps faster. It's very rare I'd consider a use case where alternatives like React Native, Xammarin, Native, Ionic etc would make better sense.

Back end I think gets more subjective especially on the database side where there's a wide range of choices that have pros and cons without a clear definitive winner. The default for databases is arguably Postgres or MySQL but those are far from a definitive choice. Mongo DB is super popular and has a big developer community but suffers under many use cases. Couchbase is arguably better but has a much smaller development community. Cloud Firestore has become a really compelling option but it depends a lot on your use case, you could be Ina situation where a graph database or in memory database makes the most sense, who knows.

Then I'd say nodejs is probably the default server side but far from clear as there's so many other good options depending on your developer experience and use case.