The author is also the founder and owner of a local Google-like company (search, email, maps, media, etc.), that's where he gets the funding for Windy.
That's against OSM's licence and seriously not cool.
This is seriously not cool.
Trivial - it's in the bottom right corner. It's easy to find if you're looking for it. The text is very small and has a bit less contrast than the one on Google Maps.
From https://www.openstreetmap.org/copyright/en
> How to credit OpenStreetMap
> We require that you use the credit “© OpenStreetMap contributors”.