Can someone explain to me how to name an ester? Like suppose we're reacting ethanoic acid with ethanol , I know we get ethyl ethanoate but how is that derived?

The IPUAC name is a bit different than the common name, but the way that the IUPAC names their esters are that they take the prefix of the alcohol and the name of the acid.

So if you react ethanoic acid + ethanol...

They take the ethyl prefix from ethanol, and ethanoic from the acid.

Hence, ethyl ethanoate.

Remember, -ic is from -ate, and -ous is from -ite.