@Abetillo While terrine has been around for centuries in various forms, sometimes (like in this chapter) an infused gelatin (called aspic) is added. Gelatin itself is usually produced by cooking what AnonDePlume mentioned, such as calf's-foot jelly, gelatin was eventually able to be dried and sold in sheets and then granulated. I think most of the time it's just spare animal parts like bones, skin, etc.
It's possible to use agar for something similar to gelatin and comes from seaweed.
However, a company named Jell-O became known for selling gelatin in the early 1900's. At some point, after world war II, mostly in the 40's-50's there was the first big jell-o mold with fruits in it. It was more gelatin than other ingredients. While it wasn't as common in the 80's, many people born in those years do recall fruit in jello which was still around.
https://www.seriouseats.com/2015/08/history-of-jell-o-salad.html
and
https://soundcloud.com/gastropodcast/watch-it-wiggle-the-jell-o-story