最后 and 终于 can be interchanged in a sentence, and they are both translated as “finally”, but their moods are a little bit different.
最后 and 终于 can be used with time adverbials.
最后can also be used with 的 to become an adjective. When it is used with measure words, 的 is usually omitted. But 终于 cannot be used in this way.