> If one company (let's say, Tesla) owned half the electric car charging stations in town, and another company (let's say, Nissan) owned the other half, and Tesla's charging stations could only be used on Tesla vehicles and Nissan's charging stations could only be used on Nissan vehicles, would you conclude that both companies have monopolies?
Yes, of course. The fact that you can't use the other company's chargers means that there are then two markets there, one for Tesla-vehicle charging stations and one for Nissan-vehicle charging stations. You can't substitute one for the other in that case, you need one compatible with your car. It's the same reason that gas pumps aren't the same market as electric car chargers, or that gas pumps aren't the same market as diesel pumps. If there is only one diesel pump in the state then it has a monopoly no matter how many gas pumps there are because you can't use gasoline in a diesel vehicle.
If you could use either type of charger with either type of car then they would be in the same market, because a customer who wants to charge their car could substitute either one for the other, so they would each actually be in competition with the other and neither would have a monopoly.
I think the fact that the "chargers" and the "cars" are hypothesized to be operated by the same company is what's messing people up.
Suppose you have two companies that each operate half the charging stations in the same region. One is Tesla, and you can only use them to charge a Tesla vehicle. The other is Exxon, which has started installing electric car chargers at their gas stations, and where you can charge any car including a Tesla. Well then Tesla hasn't got a monopoly on anything, because any Tesla owner can go charge their car at Exxon, and Exxon doesn't have a monopoly for Tesla vehicles, because they can also go charge their car at Tesla. But Exxon does have a monopoly for charging non-Tesla vehicles, because if you have a Nissan or a Chevy, you can't use the Tesla chargers, leaving Exxon as your only option.