With the major improvements in all technology I figure it's time to get a quick lesson and recommendations
of them.

I have two houses and both have thick walls of plaster and metal lathe over wood frame. Some walls are 6" thick or more.
In some cases I might like to know about detectors that are able to penetrate even more than the walls above.

I want to install into the rafters some screws which should be put exactly in the center of the stud (rafter).

Other cases I want to be sure that I've hit the center of studs where shelves might go. I'm just beginning my work on the
2 homes so whatever I get will be used a lot.

(1) At one time the way to detect a 'stud' was to look for the nails in it. So a detector would employ a magnetized needle.
A compass would be similar. But a good knock could find the stud as easily by listening to the sound. And then tapping
in with a long small nail would be the final test to see if you actually hit it.

Metal lath will only further cause confusion.

The detectors are basically measuring the density of what is under the detector.
An increase in density should accompany a stud.

You start the detector on a location that does not have anything in the wall, zero the device, then slide it over the wall.
If the density increases it reads that as a stud.

Metal mesh will kill the sensitivity of the detector, as does thick plaster walls.

To make matters worse, many old houses with plaster walls do not have the tightly controlled stud spacing required now.
While the nominal spacing is often 16 inches, a stud would be moved slightly if needed to avoid adding another stud.

Better three studs over 36 inches than 4 studs was the attitude.
If the place has full size 2x4 studs (rough cut and not surfaced) it is not really a structural issue.

The detectors work well on 1/2 inch drywall, OK on 5/8, and often fails miserably on 3/4 inch.