Serial/USB with Atmega64

I am quite new to electronics stuff and microcontrollers and am trying to hook up an Atmega64 to a USB port for serial communication capabilities with a Raspberry Pi. From studying the Arduino UNO schematic, the board has two distinct microcontrollers- the Atmega8 and the Atmega8u2. It is my understanding that the Atmega8 handles traditional functions, while the Atmega8U2 handles serial and the USB connection. Is this information correct (my research may not be accurate)? What microcontrollers would you recommend to handle the serial/USB stuff for the Atmega64? Thank you so much!


Current version of UNO R3 has 328P as main MCU while 16U2 is used as USB/Serial transceiver. 16U2 is similar (same family) to 8U2 with twice bigger memory. ATmega8 is very old in compare with 328P but they are also similar for this UNO functionality. ATmega64 is bigger one.
Your understanding is correct.
You can use 8U2, 16U2 as USB/serial. Also, parts as FT232RL, CH340 or similar which are exactly USB/serial transceivers and no firmware is required.
Personally, I prefer FT232RL because it requires minimum external parts and it goes with default setup, which is perfectly feasible for Arduino use. There is older version of UNO with this, Duemilanove I think. CH340 needs more parts include external crystal, but is cheaper than FT232RL. CH340 is frequently used by chinese clones. 8/16U2 will need the most of external parts and also firmware. So, from this view it is the most complicated to use this.