I live in a townhome built in the 80s with crazy good noise isolaton, i assume it's easier with a townhome because they just run a thick firewall between units.

Still, I would think good noise isolation must exist at some condos. I imagine you could interview current residents to see how good the noise isolation is before purchasing.

It's entirely possible. My friend told that his almost newly built apartment had very good insulation. My parents is good as well, only noise really is the elevator if on other side of wall. My current rental isn't bad either. Though some noise get through. Last place was horrible though, specially with dog that made noises through the night...

So it is possible to have good noise insulation. In general the noise from outside is bigger problem than from neighbours. Only real case with issue is renovation as any drilling or hammering through travel in structures.

I think good sound isolation mostly comes as a side effect of concrete housing, or things like firewalls in the american housing market. I think it's a side effect of how house price = sqft so all the other important things go by the wayside.

When I looked up soundproofing, there are techniques that involve a few layers of drywall and sacrificing like a few inches of air space to have an airgap, so it is possible to do it relatively cheaply in new construction, but then you sacrifice sqft :/