Use "which is". It refers to "understanding". You need to capitalize 'I" the first two times in your question.
Student or Learner
"I have gained a better understanding of process A and process B, which is/are very crucial for performing event C."
Did i use the nonrestrictive clause correctly? what i meant is the understanding of process A and process B is crucial for performing event C. should I use "is" after "which" because it's "a better understanding" or "are" because "process A and B"?
I wouldn't say very crucial- it's crucial or not- there's no need to modify it.