I would add, only from casual reading on the subject, that the bunker needs to be at least 150feet below ground. The problem there being that no one can know for sure how far they actually are from a potential targeted site, or a non-targeted site that gets hit due to error. So it's sensible to progress on worst-case scenarios because there is one chance only to get it right.
Also, a separate dome constructed above the entire top adds considerably.
The logistics of going underground are mind-boggling. An all-out nuclear war would render up-top dangerous for many years in a targeted nation, such as the USA or any NATO member. It's just not do-able for the average person. What gets sold are imperfect and unsustainable shelters that are basically tombs that you get to hang out in and prepare for your death.