Your question is quite broad / open-ended. I am not as familiar with Switzerland but in the Netherlands there are number of big electronic trading firms/market makers, e.g. Flow Traders, IMC, Optiver and Tower Research, as well as a few smaller shops, e.g. 323 Trading, All Options. There is also a special tax ruling for expats: https://www.iamsterdam.com/en/living/take-care-of-official-matters/highly-skilled-migrants/thirty-percent-ruling.