Different Cmakelist.txt

Hello everybody
I am digging in the PX4 code and I am coming across difficulties in understanding the software the organization of the code.
One of my questions is about the cmakelist.txt files. There are many of these files and I do not understand what are the differences between them.
I am using QT creator, and I am using cmake to create and run the codes. When I have to open a project I use the camakelist.txt that is in the root of the firmware directory, but if I try to use, for example, the cmakelist.txt that is in the px4_simple_app directory it does not work.
I would appreciate it if someone could give some help with that.
Thank you in advance guys