There are people on these forums who use QB64, most notably Clippy. However, you will find more of them at the QB64 forums.
Both the QB64 forums and QB64 wiki are good sources for a lot of information.
I have started using QB64, but I am not very familiar with it yet, or even any BASIC compiler. My BASIC background is interpreted QBASIC 1.1.
Note that QB64 uses MinGW (Minimalistic GNU for Windows). You can find its files within QB64\internal\c. It includes C and C++ compilers, an assembler, a linker, a debugger, and numerous header files, among other things. However, QB64 runs the GNU C++ compiler for you, so you wouldn't normally need to run it yourself.
Regarding compiling the program from the command line, see:
Regarding modular programs:
If you for some reason your still want to break your source up into multiple files, you can probably use the $INCLUDE directive.
Note that QB64 does allow the use of C header files, and external DLL files, including the Windows API. You can use DECLARE CUSTOMTYPE LIBRARY and DECLARE DYNAMIC LIBRARY for those purposes.
In some cases QB64 will try to compile a program, and the GNU C++ compile or the linker will find a problem. All QB64 tells you is "C++ compilation failed.". If that happens to you, you can follow chronokitsune's instructions here:
Doing so will allow you to view the error messages from the compiler. (I'm not sure if it shows linker error messages.) But once you have debugged it, make sure to remove what you added to makeline.txt, otherwise QB64 will continue to report failure even after you have fixed the error.