Toyota pretty much made the engine anyway.
The story goes BMW thought it was good enough, but when Toyota's engineers came in they didn't think so. So re-designed most of it and put it through much more extensive testing. Which is why it's vastly better than BMW's previous 6cyl
Apologies if I missed the thread of this, but are you talking about the new I6? If so, I don't think that's the case. Most of the parts used in the Supra-specific B58B30C (supplied by BMW) are, unsurprisingly, straight from BMW's catalogue and interchangeable from other models.
An associate of mine wrote a very comprehensive story on it that broke it down but, in short, it's effectively the same as the engine in the new X5 barring a few parts:
Cylinder head: straight from the X5, complete (inc. exhaust cam, intake cam shared with other BMWs, inc. 340i)
Intake and charge-air cooler: straight from the X5
Crankshaft: X5/Z4
Con rods: X5 (standard BMW fare, though)
Oil pan: Stock BMW 3.0 from 2015 on
Water pump/thermal management module: X5
Fuel rail: X5
Injectors: Standard BMW
Throttle body: X5
ECU: X5/Z4
Starter: X5
Alternator: X5
Turbocharger: Euro-spec Z4
Some of those may have since transpired to be also shared with the Z4 but its engine does have some different parts and is also more powerful; the Z4 has a different cylinder head, different pistons and different rods (and mapping to suit, no doubt).
It's a similar story throughout. Wheel hubs and bearings? 5 Series. Steering column? X3. AC system? X3. Etc.
Bespoke key parts? They're not huge in number for the Supra but include the oil pump, steering rack and engine mounts.
https://jalopnik.com/the-2019-toyota-supra-is-a-bmw-parts-bin-car-1829563030
Also, it's reported that Toyota (according to the head of Gazoo Racing) only developed the suspension elements, transmission and differential aspects of the new Supra. I'm sure there might have been some input on the powertrain front but BMW will have been working on it for a long time.