My Apple trackpads don’t click when they are powered off. It just feels like pressing a piece of glass.

You can also detect pressure by just measuring how much of your finger is touching the pad. A small spot means light pressure, a large spot where your skin is flatted out more against the glass is higher pressure. In practice these kinds of measurements are fairly tricky so they don't get used very much.

This. If you ever look at apples raw input touch privateframework it provides enough data for the relative direction/size as well as has some encoded states to at least detect between a very faint touch and a finger on the trackpad.

This I think goes back to the first unibody MacBooks from around 2008.

The multitouch trackpads actually showed up in the early 2008 MacBook Pros that were the last ones before switching to the unibody enclosure (IIRC the keyboard/trackpad parts could be retrofitted into 2007 MacBook Pros). The early 2008 models still had a separate physical button at the bottom of the trackpad. The late 2008 models got rid of the button and made the whole trackpad hinged at the top to act as a button. Then in 2015 they introduced the Force Touch trackpad that fixed the awkwardness of needing more force to click the further up the trackpad you went.

Yep. And this blog post from 2009 was a big inspiration for me in exploring touch on the mac: https://archive.org/details/2009-03-28-steike-code-macbook-m...

and a comment a few years ago by me: https://news.ycombinator.com/item?id=23274577

They barely deflect, strain gauges measure the deflection and trigger the haptic part. When off, it does feel pretty solid. When on, the effect is very convincing, you would never know it's not a button without being told. I had a coworker once that was ready to open up his turned-off macbook because he had been near sand recently and was convinced sand must be trapped inside the trackpad because it "didn't depress".

You can choose three settings instead of an arbitrary level of adjustment with the old screw system. It used to be super easy in the very first round of unibodies, practically tooless except for the screwdriver on the trackpad since the battery came out with a finger latch at the time. Just shut the thing off and try it out, if it had cushioning you'd be able to tell then but its basically a slab of glass.