I don't know the nuances of the terms or rules, but RB bought the engine design/technology/IP from Honda and is now officially considered an engine maker (Red Bull Power Train). They already build their own chassis so doesn't that make them a "works" team ? Yeah, I realize RB isn't an automotive brand and you can't buy a Red Bull car so is that the difference ?
Actually, upon further consideration, it's even more nuanced that what I said earlier, and the Red Bull situation is an odd duck as well.
Traditionally, a works team meant it built the whole widget, like Ferrari, who have done so almost from the start (after starting out running Alfas).
So, starting in 2026 RB would fit that definition since they'll have an engine of their own design and build to go along with their chassis.
But the term has also evolved to encompass teams that are supported, financially and technically, by an OEM, like Porsche Penske in sports car. Penske represents Porsche, but is still an independent organization, being paid and supplied by Porsche to represent them in competition. Penske is the works team, while Proton, JDC-Miller, and JOTA are customers.
Red Bull's purchase of HRC's IP and UK facility, precipated by Honda's withdrawal, doesn't really fit the mold IMO, reinforced by the fact that the engines are still built by Honda in Japan. RB is not carrying Honda's banner, except in a minor sense, nor are they receiving financial support to be "Honda's" team. It goes deeper than a customer relationship, but isn't a full works relationship.
Come 2026, RB will have its own engine, with technical input from Ford, so it may be a works team as RB, but only in that sense, not as Ford's works team, or primary representative, which is the angle I saw it from earlier, without taking into account the traditional definition. RB's initiative comes from within, not from Ford, who may contribute to the effort, and reap that benefit from associating with a winner, but is not driving the effort.
Starting then, Aston Martin will be carrying Honda's official banner, as their appointed works partner, despite the oddity of AM being its own brand, albeit not in direct competition, while their road cars rely on Mercedes powertrains.
With Sauber, Alfa has at least been upfront about it being strictly a sponsorship arrangement, not any sort of technical deal, which all will go away once Audi makes their grand entrance, and take majority control of the team. At least if the rumors about them having cold feet prove untrue.
Clear as mud, but the checks clear, which is ultimately what matters I suppose.