Cameras of that era accepted film that was slightly larger than the delivered prints. Work was done in a dark-room. Since both the camera film and the print paper were sensitive only to violet and blue light, work was performed under quite bright red light. Red light is void of violet and blue.
The film was immersed in a series of chemicals and the results were black & white negative film.
The now processed and dried black & white negative was placed in a frame not unlike a modern glassed picture frame. A negative was placed in the frame directly under the cover glass. Next photo paper was placed in direct contact with the negative. This sandwich was held tightly against the glass by a pressure plate.
A white light bulb was briefly allowed to play on this frame. This exposing light traversed the negative and exposed the photo paper underneath. The now exposed photo paper was then developed by dipping it in a series of chemicals. The result was a positive print on paper.
Photofinishers devised clever machines, usually foot powered somewhat like a manually operated sewing machine. If the sandwich was not tightly pressed together, some exposing light would leak by the edge of the negative. Additionally, some of the plate glass covers had black paint borders. This allowed sloppy negative/paper alignment. The painted borders produced rounded corners, some made prints with oval images.
My career, photofinishing, spanned more than 55 years. In the closets of many of these shops were dust covered contact printers. In other words, I have seen and touched these beasts.
The photo paper came in cut sheets or long rolls. The finished prints were hand trimmed. Various thickness of paper was available. The thinnest was just above tissue paper. This thin paper was called ADD. I think this name came from the newspaper business, thin paper prints was used when laying out the pages of the days newspaper.