I don’t think you know what “decades” means.

Headphones have a band that goesyour head (or some go around the back of your head) and sit on or over your ears.

Earphones go in your ears, but not into your ear canal. Think of the old school Apple earphones that came with their phones/iPods etc. (on the right in this pic: https://media.wired.com/photos/5932a9a458b0d64bb35d3252/master/w_1280,c_limit/09122012-IPHONE-NEW-EARBUDS-040edit.jpg)

IEMs go in to your ear canal with a little nozzle. These, for exmaple: https://m.media-amazon.com/images/I/61HketTXYcL.jpg The black bit goes in to your ear canal.