One you have some means of desoldering/resoldering a qfn, the easiest way to become familiar with it is - use junk PCBs, every piece of throwaway electronics can help you get better at soldering..
To desolder QFNs, heat them up by your preferred means, (hot air with a small nozzle works well, another way is using bismuth solder like chip quik..
You remove the chip with tweezers, or some people use a suction grabber
clean up the excess solder around where it was, reapply some flux and then practice putting it back.
Notice how when everything is properly warmed up and liberal amounts of flux has been applied - if you move the chip a tiny bit off where it should be, the chip just sashays right back into place on its own because of surface tension. Thats what you want to have happen. To do that properly hot air is almost essential. Drag soldering can be done as well but if you dont have hot air its easy to have the chip be slightly off the optimal position and the self-correcting aspect wont be available because the chip will be anchored down with the first pin and its position wont be able to be adjusted unless you laboriously desolder it with Chip Quik or similar. Preheating to a temperature well below the melting point of the solder (100 degrees C is maybe even too hot) thats used is helpful to make the process more forgiving but you cannot use a preheater for spot desoldering because once it gets hot enough to melt the (RoHS) solder especially, literally everything on the board will then come off. So use hot air to control what melts and use the preheater to make sure the parts dont undergo the solder melt and thaw too many times because that causes mechanical stress and reduces reliability-
You especially don't want to use too much temperature for too long around active components, multilayer boards with vias going through them, and multilayer capacitors. If the FR4 changes color slightly becoming glassy its too much.
I have never done a BGA so I can't help you there.