yep a cad program is where you design your model and a slicer - does exactly what it says.
It slices the model into really thin slices (typically 0.1-0.4 mm height) and then takes those slices and creates the g-code instructions that allow the mechanical 3d printer to turn molten beads of plastic into solid models.

In short the slicer is probably the most important piece of software involved in 3d printing.

Of the free slicers you can get that I have tried. I'd say that prua slicer is probably the easiest to use and most comprehensive.

I mainly use simplify3d - which you have to buy. I'd say it's still worth the money.
But If I started 3d printing now - as opposd to 7 years ago - I'm not sure I'd actually buy it. Plus when I bought it, it was almost half the price it is now.

There are printers that can (sorta kinda) use plastics and metals - but bizarrely they now cost more than a dedicated metal printer. And the post processing rigmarole is a real Pita.
Desktop metal and markforged make them.

xactmetal make the cheapest dedicated metal printer.

Oh yeah try and start thinking in millimetres. Pretty much all the software you will use will be in mm. Certainly the slicers are.

So you're looking for a 400x400x400mm build volume. Yes that is large :-)

Not all 3d printers use gcode, most do bit there are still manufacturers who like to lock you into their own software ebvironment.
Flashforge machines still use a different format. Mainly to stop people usig their slicer who don't own their printers. And it's a great slicer too.