well, I would think IG would partner with somebody that would provide more of a service than just handing the car to you.
It won't just be a hand over, the dealership will be a full fledged dealership like you walking into a Porsche dealer or a Nissan dealer (sales, finance, parts, service, etc.).
I've gathered that to be eligible for the franchise the "dealer" must have a handful of other brands that they sell, of similar clientele, as well as a long list of requirements (including after sales support, events, and so on).
As far as working on them, it has specifically been stated that those repairs will be done at Ineos dealerships only, not at BMW. Likewise, if you took your X3 into a Toyota dealership and said "you use the same motor in the Supra, fix it", they'd most likely laugh at you and help you Google the nearest BMW dealership