This functionality was first introduced in Tech hotkeys and is probably one of the most clever things ever implemented. As any player from before Definitive Edition knows, it is possible to queue units but not technollogies. However, the units tab has a field called “Initiates technology” that at least can be used to keep track of the buildings required to age up. Most actual units whose production we can queue have this field disabled, all except for the Trebuchet.
That’s the idea: creating a bunch of copies of the Trebuchet, that can be queued but also can be assigned a technology to research upon completion. Assigning each unit the icon, position in the menu, cost and research time of the technology it represents, we can mimic the idea of researching techs when in reality we are creating units. Moreover, this trick allows to have technologies with zero research time but that have to be manually clicked. Definitive Edition claims to have Loom research instantly for Goths when actually it has to take at least one second in order not to be an instantaneous tech (see the section on the Techs Tab