If a Wi-Fi and a USB-cable is connected at the same time, will my network be more stable and faster? I am trying to connect my Mac to a USB-cable and a WiFi at the same time. Will this make my network faster and more stable? Or will it just only pick one network to use?
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.