Where to find info about mbed mbed_app.json overridable parameters?

The first resource can have a look at is the platform configuration option page. Additionally, check the manual on how to use the mbed CLI to show configuration options.

Additionally, you can look at targets.json on GitHub:

For example, "Target" => "config" => "default-adc-vref" would need to be entered like this into mbed_app.json:

    "target_overrides": {
      "*": {
          "target.default-adc-vref": 3300