Focusing in mirrorless can be interesting.
Since AF in SLRs(or specifically passive TTL focusing-ultrasonic or IR active focus in P&S cameras was in use earlier) became practical in the mid to late 80s, phase detect focus has been the dominant method. In something like a Nikon F4 or any other camera of the area, you have a 3 or 5 pixel CCD sitting behind the reflex mirror-usually on the floor of the mirror box. There's a semi-silvered patch in the center of the reflex mirror that bounces light down onto the CCD. The AF module then works together with the motor to maximize the contrast between two adjacent pixels. That's also why you need an area of good contrast.
Over the past 30+ years, phase detect has been refined and improved tremendously where it's now lighting fast and precise. Modern high end cameras now have stupidly high numbers of AF points(I think the D850 has somewhere around 150). The low light capabilities have also been improved, as well as other things in in less-than-optimum conditions.
For years now, going back to P&S digitals, now cell phones, and high end interchangeable lens cameras we've the development of contrast detect AF technology. It essentially looks at the whole sensor, picks out what's important, and then maximizes the contrast in the area. It has the ability to be incredibly precise, but is also slow compared to phase detect. It's getting better, and does have the ability to focus over the entire imaging area(a big step up from the single center point in an F4). Still, the speed is a big turn-off.
A co-worker has the first generation A7R, and one of his two big complaints is how slow the AF is on it. Admittedly that's a fairly old camera as high end mirrorless goes. Incidentally, his other complaint is that it's as loud as a DSLR, something that's been addressed in newer cameras.
With that said, I admit to not having read up on it a HUGE amount, but as per that same co-worker the current high end Sony cameras actually use a separate phase detect system for course focus and then switch over to contrast to do the final "fine tune." That seems to me like the best of both worlds.
