Extending wood studs or adding wood to cover over the studs

That wall system is a “sound control” system. There are special metal framing members running perpendicular to the studs. They are secured to the studs that allows an air space between the wallboard and stud.

The metal stripping is called “RC-1 channel “. You can google it.

You are guaranteed to have studs behind the drywall. How else would the drywall stay in place? Take a rare earth magnet and run it along the wall until you hit a drywall screw or nail or if you can see a slight dimple in the face of the drywall you'll know a stud is there. Probably as other people have noted this is a party wall and there are two sets of studs, one for your side and one for the other side, that help with noise transmission.

You could use toggle bolts and likely never have a problem with 75 pounds, if you look on the packages they rate each toggle at 40lbs of direct pull our or 75 pounds if the force is perpendicular. I personally wouldn't go that route. I'd find a stud and set my mount to it. If the stud doesn't sit where you'd like to center the tv then either go with the sheet of plywood that is secured to the studs or open the drywall and add a cross piece.

The gap between drywall and studs may be to help with soundproofing - the solid stud to drywall connection conducts sound much better than a thinner connection - but there must be something somewhere holding up the drywall.

You really have a number of possible fixes. The variation is basically how much wood, how much drywall and how much patching/painting. Since this will be covered by a TV, plywood plus a couple of coats of black paint should work quite well - it doesn't need to be the kind of perfect smooth finish of a regular wall since nobody will be looking directly at it.

• Pick a rectangular area going across at least 2 studs (typically 16" apart) that will be covered by the TV. You can drill small pilot holes with a long bit to make sure you can get to the studs so you don't rip apart the wall in a bad spot.
• Cut a strip of drywall horizontally from one stud to the next with the height matched to the width of the wood you are going to use (e.g., 3-1/2" for 2x4). Measure the depth needed to get from the studs through the drywall. If it is a 1" gap + 1/2" drywall then 2x4 (1-1/2" thick) will work perfectly but if the gap is bigger then you will need thicker wood or two layers.
• Cut 2x4s (or other wood) to length and screw into at least two studs.
• Repeat at a second spot lower (or higher) on the wall at least a few inches away but preferably close to the full height of the TV.
• Cut a piece of plywood just big enough (perhaps an inch or two larger all around) to cover the 2 strips of wood.
• Paint to match the rest of the wall.

The great thing about plywood is you can screw into it anywhere - you no longer have to worry about whether you are over the studs. This is normal for mounting telephone & networking equipment in wiring closets and will work well for mounting TVs too.