Using engine to build an app

Is it possible to use your engine to build an app like Blender> and if so, how?

If you mean building an existing, external application such as Blender, then no. Blender is written in Python and has an entirely different toolchain.

However, if you mean writing your own 3D modeling application using Flax, then yes! That is very much doable, even if it involves a ton of work. As for how, it would pretty much be the same as a game. Here is the documentation. The major differences will be which Flax features are going to be useful.
e.g.

  • GUI: Heavily used
  • Navigation: Not used at all