If a Wi-Fi and a USB-cable is connected at the same time, will my network be more stable and faster?
MacOS can have multiple network connections, via WiFi, Ethernet, Thunderbolt and indeed USB.

Normally, one network takes preference over another: if the protocol can't be established on one network, it will look at another. In this way, it's possible to have Internet, file sharing, and other network devices like printers all on separate connections and networks.

Your internet connection will come from only one network. You can't combine two separate network connections and use both simultaneously.