My experience of D&D-family games seems to be unusual. I started in a university group in 1979-83, where all the DM's worlds were of their own design, and all connected. Characters could be played in any of the worlds, and could be moved between them via the "Halls of Teleportation", an NPC organisation that had facilities in large and medium-sized cities and transported adventurers for free, although they charged merchants shipping goods in bulk. This meant that characters weren't in fixed groups, and long-running "campaigns" were rare.
Players would have many characters each, at a wide variety of levels. Parties were formed for adventures, which might take a single session to play, and rarely more than three (although sessions could last all day at weekends). Plot arcs could exist, but participation in them was basically voluntary for characters, who'd go on the relevant adventures if they wanted to. This had the side-effect that many adventures were organised by the PCs interested in a particular plot.