Typically, core instrument software is located in "kroot" which is usually located or linked at /kroot on the instrument computers. All the "released" (compiled) code specific to the instrument is there. There are exceptions however, some instruments have scripts in the bin directory on the eng account or sometimes another account. A few scripts common to multiple instruments may be located on a shared directory in /sol/apps/kroot/summit/instr/bin (or other symlinked names).
The released code will typically be in /kroot/rel and its subdirectories. Where source code lives can vary from instrument to instrument. One pattern which exists is to have either (or both) dev and bld accounts which have their own kroot (typically in their home directories) and which have a src subdirectory under that kroot. When the software is "released", the new code in the src directory is compiled (if necessary) and then copied to the /kroot/rel directory which all account see.
There is variation in the release process, but the basic ideas described here should help you begin to navigate the source code.