What's the kind of construction thats used in above sentence? I understand it as depending on us but I am not sure. Does this only work with "it is" ?
"It's on us" means we accept responsibility for it. The subject isn't limited to "it". If the subject refers to something just discussed, "it" or "that" could be used. If the subject isn't clear, whatever "it" is could be named.
It could be used to accept partial responsibility. For example, there is mayhem at a party causing lots of damage, but "the exploding soda bottles are on us", meaning that we're responsible for that piece of it.
The phrase doesn't apply just to "blame", but other forms of responsibility as well. For example, several couples go to a restaurant for dinner and one couple offers to pay for everything. They could say, "It's on us" to indicate that they will take responsibility for paying the whole bill.