Pi-Apps is written in bash. This is a scripting language for Linux, with origins in the 80s. Bash is not a compiled language like C, it's an interpreted language, similar to Python and Windows .bat files.
Bash serves a different purpose than a compiled language: orchestrating OS-level events and prioritizing programming-time over execution-time. It is system-dependent and architecture-dependent.
You probably interact with bash without even realizing it! Nearly all Linux distributions today use a bash terminal. If you have ever opened a terminal, you have interacted with bash.
Pi-Apps is comprised of bash scripts. These are text files that are filled with bash commands. To illustrate this, you can often open a bash script, copy the contents, and paste it into a bash terminal. And it will work exactly the same as if you executed the file! In fact, if Pi-Apps was reorganized into a single standalone bash script, you could copy and paste the entire thing into a terminal and have a working app store!
If Pi-Apps is only bash scripts, how does it display a GUI? (graphical user interface) Good question. When Pi-Apps was just a concept, I knew that bash was the obvious choice for installing apps. And that makes sense: if you normally install an application by running commands in a bash-terminal, it only makes sense to use a bash-script.