Made changes to project solution, modifies engine source code?

Made a project out of modified source 4.15.1.

Made changes to cpp file in the project solution. Rebuild it and the changes are visible in the source code solution.

Do changes to project solution become permanent changes to the engine source code it was generated with?

The engine is shared between all solutions using it. So if you change something in the engine in one project, of course it changes in all the others using that engine too… not sure I understand what the question is?