I can say from my own experience that 5G works on the Swisscom network with the Poco F3.

If the subscription is designed for it, it will work with it. I don't have a Xiaomi Poco F3 myself (I'm not ready yet), but the 700 MHz and 3.5 GHz frequencies used by Swisscom are covered and supported by the Poco F3. It can of course use many more frequencies, but yes 5G is possible with Swisscom, Salt and Sunrise in Switzerland.