A question : How did you pop a window to select file when I type rz in the terminal

Just like the title. I read the source code of lrzsz but still dont know how to get some kind of signal or id to pop a window. I know lrzsz is based on zmodem, but is that matters?
I'm going to write a serial port terminal, similar project like secureRT can pop a window too but it is a commerical software.
Also lrzsz in pure c is way tooooooooo much hard to read, also I dont know how to debug a Makefile&configure building project. So I created a cmake version of lrzsz, after thousands of bugs I made this project to be compiled. But emmmmm its not working, it poped window in windterm but not sending.
Have a good day man!! Thank you for such a good terminal project!

The issue names no repository files, tests, or entry points. Start by locating the terminal's lrzsz or ZMODEM integration and the CMake-based build mentioned in the report; the work is not complete until file selection and transfer both function, but the required behavior is not specified clearly.

Written by the indexing model from the issue text.