How to edit geometry in the deep narrow concave parts of mesh?

For example, this mech hand:

enter image description here

I imagine there are many tiny faces in these grooves:

enter image description here

How do people edit the topology there? Or this kind of model is usually modeled as several discontinuous parts?

Yes, this would be modelled in separate meshes within the same object or as separate object.
It can be done as one mesh if you need it and use hide / unhide the part of mesh you working at.

But if it is separated in real life, which i would guess at this, you would separate it in blender too.