Designing stuff: FreeCad and OpenSCAD. Lots of YouTube material for educational purposes.
Regarding the printer, this depends on the make/model/manufacturer, but the more serious people (not starting a flame war here, just my observation) tend to use Klipper. Most printers run some form of Marlin, but most can run Klipper as an alternative firmware.
And there are the slicers… A huge amount of choice here, but one way or the other, most are forks/spinoffs of previous work. Personal choice here is SuperSlicer, mostly due to its interface and me being too lazy to use OrcaSlicer of one of the other options.
But beware: tuning, measuring adjusting will take a lot of time. And the entire 3d printing community likes is.