You take a photo of an event. The event is recorded as a negative, originally on glass plates before camera rolls became the standard. It's called a negative, because how light sensitive chemicals are used, the bright things burn it dark, and the dark things are barely recorded as clear. So White on Black events is recorded as Black on White.

Then to get a print on paper you soak paper in light sensitive chemicals in a dark room. Shine a light through the negative onto the white paper, and gradually the light sensitive chemicals will darken to black. Let it dry and your Black on White negative becomes a White on Black photograph.

Now to modify things:

To crudely delete people from black and white: Scrap/wipe the persons image from the original negative to create a white patch on any future print. Or Paint/Airbrush over the person with paint on the print, and photograph the finished photo to get a clean negative.

To add people: Take 2 prints, "cut" out the person you want from one print with a knife or scissors, and "paste" it with paste into place on the other, then rephotograph to get a clean negative.

Another way to add people, is to print out a print that is the same size as the negative. Then place that print over black cardboard and cut out what you want to keep with a knife. The original cardboard is now split into two "masks". One mask is of what you want to keep from the first negative and the other mask's of what you don't. Place the keeper mask against the first negative and shine a light through it onto a fresh print, so get a partial print with blank spaces. Then put the other mask against the other negative and shine a light through it, so the print now has the blanks filled with the other negative.

To make things bigger or smaller in relation to each other: The further the print is away from the negative during the printing process, the bigger the negative image that will be projected onto the paper. Once you have two differently sized prints, either cut/paste or mask them into a final print.