If you want something out of box for free you can use Blender and download rigged models here: https://sketchfab.com/3d-models/rigged-characters-482b7772d3.... Or find poses and animations from https://www.mixamo.com.

Or you can use Houdini's free version, it comes with a fully-rigged mannequin and a realistic proportioned human. You can't export them in free version but if you're a 2D artist you can just screenshot and use it as a reference.

Houdini has everything from full-body muscle simulation to writing a OpenCL image processing kernel there. All free if screenshot-as-export is acceptable in your use cases. If you need actual rendering it's $25/mo billed annually.

This website's pro plus subscription is $15/mo.

(Not saying there is no room for another tool. Just stating the alternatives.)

You can also just upload models to Mixamo and pose them using their animations right in there.

However I suspect none of these (at least for static posing) beat the usability of the solution this app is imitating: purchasing a portable, posable mannequin that’s built for purpose.