In the usual case, متي is used as a question tag with the meaning of "when". while عندما is used with the meaning of "at the time of". But there are some cases when they are both used as time conjunctionsthe, where difference between both is very subtle and could even be replaceable. The main difference would be that متي is often used to consider the time in the future (near or far), and rarely used with the past. While عندما is used with the past, but could be equally used with the future as well.

متى : تستخدم للسؤال (It is used in question form) عندما : تستخدم للإجابة على السؤال. (It is used in answer form) For example: متى تذهب إلى العمل؟ When do you go to work? -عندما أستيقظ -After waking up.

متى is the question word متى تذهب الى العمل؟ when do you go to work? عندما is used to connect two sentences for example:I saw you when I was driving by school رأيتك عندما كنت اقودسيارتي بالقرب من المدرسة