Complete noob here & I'm in the process of installing Openwrt on this network switch & i had a couple of questions.
Is this the correct type of UART cable i need to use to flash the device i don't want to brick it with the wrong voltage this one seems to have the right 3.3v voltage & if my understanding is right i don't need to connect to the VCC pin on this device just the three other pins.
yes, the 3v3 variant - but the price is pretty steep. From the slow ship those would be ~1.30 bucks, locally (EU) around 5-10 bucks, depending on the details. cp2102, ch340g, pl2303, ft232rl, whatever is cheapest (ft232rl is often counterfeit, the windows drivers tends to reject those (so on windows you'd want the genuine article), the linux drivers don't really care), just make sure that they are explicitly meant for 3.3V. Having a second -cheap/ different- alternative is usually better than overpaying for the 'perfect one'.
that screen command is all you need
personal preference and familiarity, I use screen, either will do
As long as you keep the bootloader and bootloader environment(s) alone, you can't do anything (fatally) wrong and can always recover easily. Just make sure that GND is where it's supposed to be and that Vcc/ 3.3V doesn't get connected, and check RX/ TX until it works.
This kind of devices (routers/ switches with serial console) only have very simple needs (primarily the voltage must match, 3,3V (the most common option, as used here), 1.8V, 2.5V, 5V - and RS232C with DB9 or RJ-45 connector would be something completely different (+/- 15V, up to 25 V)), they don't need any of the advanced features that might be required from the usb2serial adapter for other usages.