All the floppy drives I've ever seen had a little mechanical arm attached to a microswitch that detected the presence or absence of the write protect notch. VHS cassettes worked similarly.
Audio cassettes had this also. A little tab you could break off that would prevent recording. If you later changed your mind and wanted to record over the tape, you would just tape over the hole.
No need. It was just pushing something inside the drive. A little piece of paper jammed in the hole worked fine too.
You also drilled a hole on the other side to make the disk double density.