difference between that and it's here

I have heard both of these used and would like to know what is the difference:
That's obvious; it's obvious.

I think the use of 'it's' works in more situations, if not all. But the use of 'that's' is not possible in all cases.

For example, we can say: it is obvious he likes her, but not that's obvious he likes her.

So, do they just mean the same and the context determines which is better to use? Is that the explanation?

Thanks.