CMake - two source files into one executable

Hi to the Community,

i have a question about the CLION IDE regarding CMake:

I read (in a tutorial) that I can add a second file to the same executable like this (and I tried it):

cmake_minimum_required(VERSION 3.30)
project(CMakeTest)

set(CMAKE_CXX_STANDARD 26)

add_executable(CMakeTest main.cpp 
				general.cpp)

I don't understand in which way they are gonna get compiled. I understood that only one can have a main() function. When I tried to use general as a “library” ('#include “general.cpp”' in main.cpp) with a sample function xy only defined in ‘general.cpp’, I got an Error saying 'xy is defined twice'. So how are they put and compiled together and what i the advantage of doing that? (Sorry if I have some fundamental misconceptions)

Thanks in advance