I think that you're using "campaign" and "adventure" somewhat incorrectly (or, at least, atypically). So is WotC, though. Or maybe the meanings are shifting a bit.
A Campaign is a series of game sessions with continuity. Typically, that means that most of the same players are playing mostly the same characters and are interacting with mostly the same world - it's the same campaign "if you can describe all of the events in the game in terms of the fictional narrative of the game" (Angry GM, about 1/3 down).
An Adventure is an individual chunk of story: an incident that occurs in which the PCs are active participants.
Continuing to quote Angry GM:
The Lord of the Rings [...] is a good example of a campaign. You can break it down into adventures like Flight from the Shire, the Battle at Weathertop, the Mines of Moria, and so on. Each adventure serves, in some way, to advance the larger story.
Then came the Adventure Path: a published set of connected adventures which together tell a story. Game publishers could publish stories for GMs to slot into their campaigns, or even whole campaigns for them to run (this is more or less where Paizo started with Pathfinder, incidentally). LotR could be an Adventure Path, but so could The Mines of Moria (telling the story of getting through the mines: the attack by the water monster at the puzzle door, avoiding getting lost, encountering Balin's tomb, fighting the goblins, fleeing the balor). A GM running "LotR: the campaign" could slot in Mines and avoid having to plan those sessions, or could even run Mines as an entire campaign (it would probably be written so that the exact reason that the PCs needed to traverse the mines was abstract so that the GM could pick those details; as a campaign, "get the McGuffin to the other side" would likely be sufficient).
So, Descent into Avernus is more appropriately an adventure path, which was designed so that it could be used as a full campaign: the first-level PCs meet in a tavern, descend into Avernus, kill the dragon, and return to tavern to regale the patrons with their tales (I've never actually read Descent, so it might not be a dragon). When that's done, the players retire those characters and start new ones for a new story in which the events that the Avernus characters experienced didn't necessarily happen.
Descent can also be used as a part of a campaign: the same-ish party could continue on to new adventures after returning from Avernus, with those events having happened to them explicitly.
I mentioned Paizo earlier: their early works included adventure paths which were, themselves, interlinked - the second assumed a particular resolution of the first had happened relatively recently. Each of their adventure paths assumed PCs started at level 1, though, so it is highly probable that there are groups which have run through multiple adventure paths with narrative continuity: the PCs from the first path retiring or going off to do other stuff after completing the first path and new PCs starting in on the second in a world where the first's events had happened as backstory. Whether that counts as a single campaign or multiple is a little fuzzy, though.
TLDR: PCs make the Campaign
Since a campaign is a series of adventures which share a continuity, a PC can only exist within a single campaign: by itself, that PC provides continuity even as it moves from one adventure to the next. And, WotC is publishing adventure paths and calling them adventures, but it's possible that they don't want to call them Adventure Paths because that's Paizo's shtick.
PCs have always existed in a single campaign and have always encountered many adventures in the process (barring the PCs who die immediately, game groups that fall apart after the first session, etc.).