My house was built in 2004. Solid walls downstairs, partition walls
upstairs. What is between the partition walls if I want to route a
cable through the space? Easy job, or will I have numerous stiffener
beams/laths to content with? How best to do this? For example, a
larger hole one end through which to poke a small (e.g. dental)
mirror, so that one can see what one is doing?

MM