Some plugins do work internally at 64 bits float processing depth (sometimes only for the more complex part of the processing), but most of them do exchange audio data at 32 bits float with the DAW host.
I think that developers try to not use 64 bits everywhere so that they can compute four 32 bits words at the same time using SIMD instructions. This speed up the process.
You can check this enabling the “show plugins that support 64 bits float processing” option in the Nuendo Plugin manager.
I think that this display option show plugins that can exchange data at 64 bits float (even if those plugins do not use this resolution internally !).
You will see that most plugins are not communicating at 64 bits float; except Steinberg ones and some rare recent ones.
This mean that as of today, most plugins are communicating with the DAWs at 32 bits float depth, with some of them eventually processing at 64 bits float internally.
Very few of them have data exchange at 64 bits float and internal processing at 64 bits.
And we will probably see in a near futur, updated version of existing plugins that will exchange audio data at 64 bits float, but that will keep their internal processing at 32 bits float ! for marketing reasons !
To be sure about a full 64 bits path from end to end in a DAW, it is possible to use a bit meter at the end of the path. Can be done in Wavelab for example. I’m not sure if something similar do exist in Cubase or Nuendo for checking that. For even more serious testing, a 64 bits float signal generator and a level meter than can display very low 64 bits float levels would be necessary. Something probably not available.
Last, i would be curious to listen for null tests, nulling the same mix at 32 and 64 bits float.