Use of “是不, ...” to introduce sentence
I checked the video at 35&quot;, it's 这不, not 是不. I don't think there is 是不 to start sentences with.
Basically, you can understand 这不 as a shortened version of 这不是嘛(isn't it [so]) which is rhetorical and means 是这样的(it is [so]).
In speaking, I think its function is just something to get listeners attention by marking the start of a point or narration. I don't know any expression in English that is quite the same as 这不. But I think there are some possible flexible translations:

这不, 一下车我们就直奔XXX
So, we went straight to XXX after getting off our cars.

Here it can be translated to ‘so‘, or 'that's why' (like they did in the subtitle of the video) because the content before that was saying the city had a long history, considering which so they went to the museum first. However, 这不 doesn't really indicate causality in general, it's just a casual piece of word to connect sentences which can be interpreted in many ways. Some times it would be better translated as 'now' or 'here'. Examples:

A: 他怎么还没到?
Where is he (literal: Why hasn't he arrived?)
B: 他这不来了么
(Now/look) Here he comes.

P.S:
I don't think it's standard Mandarine, it feels more a northern (Beijing/Hebei at least) dialect to me. The whole video is narrated in such style to be more approachable, I guess.