Yes there is. It's called anti-trust law and a large part is about pure market share.
It's also illegal to use your monopoly in a market to get advantages in that market. Most notably, you can't use your monopoly to keep your monopoly. (I.e. intel making a deal with dell where dell cannot use AMD processors)
It's legal to have a monopoly and get advantages in the market.
It's illegal to have a monopoly and then try and extinguish your competition in the current or any adjacent markets.
My given example of dell and Intel actually happened and was prosecuted. How does that fit your claims?
Monopolies and oligopolies are inefficient. Whether legal or not they result in in detrimental outcomes.
That's not a law against having one, only abusing one.
No, it's still not illegal to have a Monopoly. The FTC can block mergers that can create them if they believe it will harm consumers, e.g. through higher prices caused by less competition, but they typically won't block them if they think consumers will benefit through operational efficiencies gained by consolidation.
I mean that’s basically them abusing their monopoly on Walmart stores right?
Let’s get things straight. Apple does not have a monopoly on smartphones. Apple does not have a monopoly on smartphone app stores.
Walmart tells a manufacturer they have to pay a special Walmart surcharge every time a sale is made, and that they can't raise their prices only at Walmart (or give discounts at other stores) to make up for the difference.
And, in addition, after the sale is made, if the product allows the customer to buy add-ons after they bring the product home, the customer is required to go back to Walmart to purchase that add-on, and another Walmart surcharge is assessed; the manufacturer isn't allowed to sell add-ons directly to the customer without Walmart acting as middleman.