I wrote up a post about Switzerland quite recently, which you can find here. To boil it down to the basics though, the Swiss were in an awkward place. The Nazi regime was not popular, but nevertheless a policy of "Renewal and adaptation" was what they followed once France fell. There was simply no other choice than to make Germany a principal trading partner. As the tide turned against the Germans, the Allies were able to exert more and more influence to lessen this of course. The Swiss experience is very controversial for several counts beyond the simple intermixing of business: The laundering of expropriated gold on behalf of the Germans; the periodic unwillingness to take in a higher number of Jewish refugees; the Swiss banks which failed to return assets to the survivors and heirs of Jewish Holocaust victims. The linked answer explores that and more in much more detail, and I'm happy to answer follow ups best that I can!