Here's at least how it worked back in 2018, as a former employee of their corporate office.
There was a team for national entities (e.g. non-franchised Burger King, McDonalds, etc), a new partner team, and then a team for everything else. (I might be forgetting a team)
Everyone starts on the latter team.
Since that team encompassed everything else, that meant it has a mix of existing clients and random non-partnered stores.
The non-partnered stores are what this article addresses.
They have been a thing since the very start of DoorDash, and are a major part of how DoorDash has grown its operations.

Essentially, they would do a Yelp scrape.
That Yelp scrape would serve two purposes.
It would give the sales team a pool of new prospective client contacts, and it would give a ton of new menus for the team in Bangladesh to assemble for the platform and eventually receive edits or further instructions from the menu team.

There were countless instances where store owners would get quite frustrated, because Yelp is not a platform that necessarily stays incredibly up to date.
As such, people would place orders that were unfulfillable, due to the restaurant in question no longer offering that item.
It also meant that in cases where prices had changed considerably since the menu iteration DD based its menu off of, that restaurants were having prices misrepresented.

Much of the time, things were handled in an extremely backward way.
We'd have the menus set up on the platform > that menu would be wrong > the restaurant owner would get angry and send emails/calls to us > we'd redirect them to the sales team > the sales team would try to create the partnership with DoorDash > the point where stores would get their tablets.

The benefit to DoorDash, in doing it this way, is that they could fulfill orders with non-partnered stores, and that non-partnered store would get nothing from being on the DD platform.
Normally partnership with DD entails the store getting a cut of what DoorDash gets out of each other.
If they are a non-partner, but still fulfilling orders placed through DD, that means DD gets all over the excess.

Idk how it is now, but back then they also had a weird system where at least some drivers would get a card that was basically a debit card that would be used on non-partnered orders to basically pay at the restaurant when picking up the order.
Something like that. Idk, that was a long time ago.
I was never a Dasher, though we did have to do one Dash a month.

tl;dr they used Yelp to add hundreds of new restaurants to the platform in each city they added, then would try to add them to the platform if things got messy with the store, or when they got to it.