Do I need to pay to see that? It is confusing.

Anyway yes calculator apps are a fantastic project:

1. Can be anything from just running a lisp repl and doing (+ 2 2) to a full blown calculation and programming environment like Jupyter!

2. Can explore parsers, ui, ux, interpreters, graphics (if adding charting features for example), stack data structure, floating point arithmetic (or alternative arithmetics like mathematica) and so on. Lots to explore!

3. It is not a money making idea really which is good. So you can focus on the tech and not get an "entrepenuerial seizure" aka "i got an idea it make meeellllliionnnss!"