Only certain Brother and Fuji Xerox printers have similar engine installed so they can use a particular compatible drum unit.

HP and Canon laser printers are both manufactured using the same engine and technology for decades since the 1970's. They are both compatible to each other.

Its because the drum unit and toner cartridge were replaced together. It can provide consistent print quality and  it is easier to install the cartridges into the printer.

It costs less to have the toner cartridge and drum unit to be separate. Because typically you only need to replace the toner cartridge every now and then. And the

The toner cartridge is a container that holds the toner powder. The drum unit is an electrically charged cylinder that transfers the toner powder to paper to create text and