AVR Freaks

And you will continue to see garbage until you use a RS-232 interface converter. The chip-of-the-day was MAX232 but there are modern equivalents.

BTW: Don't connect TX from the PC directly to Arduino or the magic smoke will be released.

Once you get the RS232 voltage levels that are coming out of your PC's Serial port to the levels of your AVR, then switch your lines so that the RX of the PC serial port goes to the TX of the AVR UART, (and TX from the PC goes to RX of the AVR).

The serial port of the PC puts out -9 to -15 volts  in serial-port idle mode (logic high__no data being sent), and +9 to +15 for logic low.  This is a legacy from 50 years ago called the RS232 standard.  A chip of the family called the MAX232 was the most common way to convert RS232 to normal logic that went into the peripheral's UART.    The AVR does have internal diodes on the UART pins that *MAY* have protected your AVR chip from the RS232 levels put out by the PC, especially since the RX of the PC was not connected to the AVR UART.  But the PC will not receive data correctly from the AVR.

Modern USB ports use normal (+5 to 3.3V) non-inverted logic for data transmission.  In USB, the two signal lines are bidirectional, have always opposing logic values (when one line is high, the other is low), and are designed to work much faster than serial ports based on UARTs.   Modern PC/USB-to-serial devices use the 25-cent CH340 chip instead of the $15 FTDI chip, which does the same thing. But USB-to-serial devices are still called "FTDIs" and sell for a dollar or two on eBay.  I recommend that you get a few and use them instead of the PC serial port.  The PC serial port is really only good for specialized equipment from the 1980s and 90s that use the RS232 standard and still need to be in service.