It sounds to me as if the speaker went off on a series of digressions and lost his train of thought. So the sentence was never completed.
Let’s take a similar sentence that might be like what he started out intending to say. I’ll add a few extra words for clarity.
So if we [were to] start where we left off in the last video, [because] we [previously] started right over here, [...] we would be looking at this.
In this case, the main “if” clause is in the future subjunctive. declaring his intent to do something in the immediate future. It’s a pretty formal way to say this, compared to, “Let’s start where we left off,“ or “We’ll start where we left off.” The second clause, with “started,” is simple past tense, stating what happened last time.
Then I made up a third clause that completes the sentence in a way that makes grammatical sense. The prof, speaking extemporaneously, doesn’t say anything like that. After, “right over here,” he breaks off into a series of digressions (“and just as a reminder, ...”) for more than a minute, and when he’s done, he never comes back and ends his first sentence. Either he’s lost his original train of thought, or he decides it would be better to move on.
Another way to parse this statement is that he starts out introducing the video one way, then changed his mind and did it another way. So, perhaps, he’s first saying, “So if we start where we left off—” but he rephrases that as, “We previously started right over here, and just as a reminder [....]” Then the first clause would be a sentence fragment. If he’d said, “We’ll start where we left off,” or “Let’s pick up where we left off,” either would have been a complete sentence and there would be no confusion. A false start like that would usually be edited out of a video, but you unfortunately can’t do that in a live lecture.