A market is the set of goods or services that are fungible substitutes for one another. If Ford and Chevy use the same kind of battery as each other but Honda and Toyota use a different kind then "battery for Ford or Chevy" is a market and "battery for Honda or Toyota" is a different market. If they each require different kinds of battery then "battery for Ford" is a separate market from "battery for Chevy", but even within those markets there can be multiple competitors, i.e. Ford isn't necessarily the only company that makes batteries compatible with Ford vehicles.
Whereas if Ford vehicles use a different kind of battery than other vehicles and there is only one company who makes that kind of battery, "battery for Ford vehicles" would be a market in which that supplier has a monopoly, regardless of whether the one supplier is Ford or someone else.
Which one of these looks like what Apple is doing?
> Ford isn't getting sued either for only selling cars with Ford motors inside.
Ford isn't only selling cars with Ford motors inside. They offer them that way, and it's a popular purchase option, but they'll also sell anyone essentially any part of the car by itself. They also don't prohibit you from taking the engine out of your Ford and putting it in a Chevy or vice versa.
More to the point, the EU have already slapped them with two separate $500m fines having repeatedly found that Apple used iOS and the App Store to break antitrust laws.
The US have a similar case hinging on all sorts of things ranging from green text message bubbles to heavily restricting access to the NFC API and affording to itself the privilege of host-card emulation via Apple Wallet.
A lot of things on cars are welded or otherwise very difficult for the customer to modify, and I don't think any court would have a big problem with that.