STM32CubeIDE: when I "convert to C++" a C project the only difference seems to be that .cpp source files are also compiled during build while in a C project they are not. Is my understanding correct?

[Cartu38 OpenDev]

As end user visible effect I would say yes.

C->C++ is mainly activating C++ toolchain. So adding .cpp support and final link process invoking g++ instead gcc