Can the knock test tell you whether a stud is wood or metal

From your description of putting a screw into the walls, it certainly sounds as if you have lath-and-plaster. Another sign is the color of the dust: with drywall, it will be very white and uniform, whereas with lath-and-plaster, it's greyish and has darker flecks.

A lot of stud finders work by detecting the change in the dielectric constant of the wall as you move horizontally across the wall. Stud finders have a hard time with lath-and-plaster because the plaster is solid and can often be 1/2" thick, then underneath the plaster you there are only small gaps between the wood lath, so you never really get the sharp change in capacitance that they look for. On the other hand, drywall is spongy so there's less material between the stud finder and the stud that it's supposed to be sensing.

I had a deep-scan stud finder, and it wasn't that reliable, but I developed a technique that I had reasonable success with: do several scans at different heights and different starting points on the wall, noting where it said there was a stud. I also found that scanning slowly was better than faster. Eventually, I would get to an "average" location that was a pretty good guess.

I later learned (at least in my house) is that there's a stud on each interior wall right where it runs into an exterior wall. I could then mark 16", 32", etc. back from that stud and be OK.

A swiveling or articulating TV mount will tend to want to pull itself out of the wall when swiveled or extended, more than a flush-mount bracket would. Therefore, this situation calls for a bit of over-building to ensure safety, and to put up with the additional stresses of moving the bracket around.

You will need the following:

• A sheet of 3/4 inch plywood, cut as wide as the gap between the studs, plus at least 3.5 inches. It should be at least 50% taller than the mounting bracket. Hardware stores sell cut pieces of plywood that are 2 foot by 4 foot, so this is an easy choice as it requires only one cut if you don't mine the plywood being a bit over-sized.
• 6 qty of 3.5 or 4 inch lag bolts, or a box of 4 inch deck screws.
• 6 or more 1 inch bolts as fat as possible that fit through the mounting slots on the bracket, plus matching t-nuts and washers.

I suggest you pre-paint the plywood to match the wall or the bracket. Either will minimize the appearance of the plywood.

Center the bracket on the plywood and mark the holes. Drill holes big enough for the t-nuts and install them on the back side. Hang the plywood on the wall and mount it with the lag bolts or the deck screws. If you are using lag bolts, make sure to pre-drill the studs before installing the lag bolts. Since I can install a screw with one hand, I often use 2 deck screws to initially hang the plywood on the wall, then drill and install the lag bolts.

If you use only deck screws, try to find ones marked with a shear force rating and over build by a factor of 2-4. Or just install at least 6 or more per stud to be safe.