Some of this can and probably will be covered in a thread specifically about IADS, or Integrated Air Defence Systems, however there are several things which some people seem to need to have pointed out.
One of the first is that in order for something like a drone or a cruise missile to be detected by a radar system, the system needs to be on, pointed in the correct direction, and there needs to be a direct path between the two. If a drone or cruise missile is flying at a low altitude, then it can potentially cross a long distance while remaining below the radar horizon, which basically means the curvature of the earth would shield the approaching drone/missile from detection until it was either close to the intended target, or a radar system en route.
Secondly, Saudi Arabia is a large country, roughly 2.1 mil. sq. km, which means that it can be quite difficult to maintain radar coverage over the entire area, particularly given the relatively low population density and dispersal of population centres. As a result of this, I do not make the assumption that there is radar saturation coverage. There would likely be a significant amount of coverage near/around certain parts of Saudi Arabia, but I expect there are holes in that coverage as well.
Thirdly, the main strike location was in Abqaiq, which is ~50 km from the coast, which means that it is hardly in the middle of Saudi Arabia. To put some things into perspective, if a LACM was flying towards Abqaiq over the Persian Gulf at an altitude of 30 m, and there was a radar system in Abqaiq mounted 30 m above ground level, the radar system would likely not be able to detect the inbound LACM until it was within ~50 km of Abqaiq. That is also assuming that the conditions are perfect, the radar system is functioning correctly, the operators are competent, etc.
As for knowing the point of origin for the launch, unless there was some sort of ISR coverage of the launch location at the time of launch... then that it unlikely to be known. Instead we have an apparent vector, which strongly suggests that the Houthi claim is false given the location of their strongholds being in basically the opposite direction from where the strike came in from. IMO it would be far more likely that the strike was Iranian in origin, either launched directly by them, or by one of their other surrogates/proxies at their direction.