I would like to know, what I can legally do with SDR / a Hack RF One in Switzerland?
What are the limitations concerning sending and receiving?
Do I need any licenses for that? If so, which ones, and for what are they usable?
The device usually doesn't have anything to do with legality, it's what you do. So, rule of thumb: Without a license, you can't transmit – full stop.
I'm not quite sure about federal Swiss law, but chances are you can only receive broadcast transmissions (meant for everyone to receive) and transmissions especially meant for you, and everything else is eavesdropping / invasion of privacy. Amateur radio transmissions are by definition broadcast, so always OK to receive, but you can't transmit in these bands without an amateur license.