I can't seem to understand the reason as to why multiple programming languages are used in the same product or software?
It is quite simple: there is no single programming language suitable for all needs and goals.
Read Scott's book Programming Languages Pragmatics
Some programming languages favor expressiveness and declarativity (a lot of scripting languages, but also high-level programming languages like Agda, Prolog, Lisp, Haskell, Ocaml, ...). When the cost of development is important (human time and cost of developers), it is suitable to use them (even if the runtime performance is not optimal).
Other programming languages favor run-time performance (many low-level languages, with usually compiled implementations, like C++, Rust, Go, C, assembler, also specialized languages like OpenCL ...). When the performance of the code matters, it is preferable to use these languages.
In practice, it is unlikely to have a programming language which is highly expressive (so improves the productivity of the developer, assuming a skilled enough developer team) and very performant at runtime. In practice, there is a trade-off between expressivity and performance.
(however, there has been some slow progress in programming languages: Rust is more expressive than C or perhaps even C++ but its implementation is almost as performant, and probably will improve to generate equally fast executables. So you need to learn new programming languages during your professional life; however there is No Silver Bullet)
Notice that the cost of development is more and more significant today (that was not the case in the 1970s -at that time computers where very costly- or in some embedded applications -with large volume of product). The rule of thumb (very approximate) is that a skilled developer is able to write about 25 thousand lines of (debugged & documented) source code each year, and that does not depend much on the programming language used.
A common approach is to embed some scripting language (or some domain specific language) in a large application. This design idea (related to domain-specific language) has been used for decades (a good example is the Emacs source code editor, using Elisp for scripting since the 1980s). Then you'll use an easily embeddable interpreter (like Guile, Lua, Python, ...) inside a larger application. The decision to embed an interpreter inside a large application has to be done very early, and has strong architectural implications. You'll then use two languages: for low level stuff which has to run quickly, some low level language like C or C++; for high level scripts, the other DSL or scripting language.
Notice also that a given software can run, within most current operating systems (including Linux, Windows, Android, MacOSX, Hurd, ...), in several cooperating processes using some kind of inter-process communication techniques. It can even run on several computers (or many of them), using distributed computing techniques (e.g. cloud computing, HPC, client server, web applications, etc...). In both cases, it is easy to use several programming languages (e.g. code each program running on one process or computer in its own programming language). Read Operating Systems: Three Easy Pieces for more. Also, foreign function interfaces (e.g. JNI), ABIs, calling conventions, etc... facilitate mixing several languages in the same program (or executable) - and you'll find code generators like SWIG to help.
In some cases, you use metaprogramming techniques: some parts of your large software project would have code (e.g. in C or C++) generated by other tools (perhaps project specific tools) from some ad-hoc formalization: parser generators (improperly called compiler-compilers) like bison or ANTLR come to mind, but also SWIG or RPCGEN. And notice that GCC has more than a dozen of specialized C++ code generators inside it. See also this example. Notice that metabugs are hard to find.
See also this & that & that & that answers of mine related to your question.
Study also the source code of several large free software projects (on github or from your Linux distribution) for inspiration and enlightenment.
PS: there are also social or organizational or historical reasons to mix programming languages; I'm ignoring them here, but I know that in practice such reasons are dominant. Read also The Mythical Man Month