The easiest way to interpret the meaning of a sentence which uses by which is replacing the part comes before the by which with "which" . something like this
Othering is a binary process between the colonizer and colonized by which the former is able to affirm his own identity, but the last is considered as the other
by "a binary process" the former is able to affirm his own identity, but the last is considered as the other
Note that I didn't use othering, is, between the colonizer and colonized because respectively it's the subject of the sentence, is the auxiliary verb (do, does, is , has have, am are etc..), the rest just extends the meaning.