The common setup is one gateway arduino connected via nRF24L01+ radio to other arduino nodes with sensors/actuators. The nodes can form a tree network with the gateway as the central point.
The RPi/computer will be the host for the controller software, eg home assistant, and is connected to one or more gateways. Often the host of the controller software is just called the controller in mysensors lingo. The gateway(s) is usually an arduino, but can also be the RPi itself. ESP8266 is also supported as gateway. The connection between controller and gateway can be serial/USB or ethernet/WiFi.
Mysensors has different transport options for the connection between the gateway and nodes: nRF24L01+ (2400 MHz) or RFM69 (433, 868, 915 MHz) radios or serial RS485 wired. You can have sensors directly on gateway also, which means you can use ethernet, WiFi (ESP8266) or USB/serial cables to connect multiple gateways with you controller/RPi.
Learn more at https://www.mysensors.org/.