The Warsaw to Paris trip on this train takes almost 21 hours... It's just a regular train, not particularly high-speed. There are a faster options available (down to 16 hours), but they require multiple layovers so I wouldn't say they're viable in practice. Overall, Europe is mostly dominated by cheap airlines, esp. in the Eastern part.
Actually, the Eastern part still has several useful and quite cheap night trains. In the west, lots are being cancelled because the train companies would rather have us take high speed trains.
Yep, but the distance that you can travel overnight is usually well under 1000 km... Which, even in Europe, is rather a regional and not a continental trip. The latter are practical pretty much only via air.
There's no official (government/public run) website, so Seat 61 is probably the best resource.
https://www.seat61.com/paris-moscow-express.htm