It's easy to assume that a megacorp owning everything also produces everything you'd need. Why would Samsung allow Apple make a shop selling iphones in their own headquarters? Also, what would a megacorp really get from such an agreement with another megacorp? Risk losing power back at home for the chance to gain some in a foreign land? Importing stuff from another megacorp to sell yourself, sure, you could assume that's what commercial pacts are for, but allowing the other megacorp to sell stuff on its own is a straight loss from their home revenue.
Capitalism is built on consumerism. Sure, Samsung doesn't want iphones selling in their neck of the woods, but they recognize that if they want to sell Samsung in iphone's markets, they need to allow it. What happens in a megacorp civic, in my opinion at least, is that Samsung and Apple would simply merge together and offer both products in each others stores. Some people want samsung phones some people want iphones, but the consumer is happier and willing to spend more money if they can choose one or the other. Forcing consumers to buy one brand would actually mean less sales, because some people would just not be into one product or the other.
Monopolies are bad for what would happen to the price of both phones, they would probably both be more expensive. But realisitcally, the price of a given product isnt determined by how much competition there is or isnt, its determined by demand. High demand drives prices, more choices creates more demand. Sell too high, then customers might decide they simply dont need a smart phone once every 2 years, but rather once every 5...
You run into problems with products that have inelastic demand, like medicines that keep you alive. If you need a medicine to survive, there is no limit to how much you will pay, but even then, selling at absurdly high prices will just mean more people will die and not be around to buy your other products. You price medicine to maximize profits, selling exactly as many medicine as you can make a decent profit with, and maybe a few poor people die but you sell enough medicine that even if the price isnt as high as it could be, you still make bank. There is a price point for everything.
But going back to samsung and iphones, how do you convince someone to buy an iphone 10 or a samsung 12? Its not because you own both brands that you will be selling them like hotcakes. its because you have added some new feature that will convince customers to abandon their perfectly adquate iphone 9 or samsung 11 and get the newest one. Innovation drives consumption, and what could possibly bring more innovation than suddenly having an entire alien civilization's worth of products to sell at your stores? Suppose you encounter a species that doesnt use smart phones, they would either want to buy a ton of them, or they would want to sell you their wearable scarf communicators. Maybe both. You could sell wearable alien scarf communicators and iphones at your store, they can do same. Some aliens will switch to iphones, some humans will switch to scarf communicators. Either way, you make money.
Remember that every megacorp employee, which is basically everyone, is interested in also being a megacorp consumer. When that happens, decisions tend to favor consumers a bit more than you would expect, if every consumer wasn't also an employee. Why would you pay your employees crap wages? Who would buy your products if everyone is getting crap wages? In our world, corps feel free to pay their own employees shit wages because they know there are other people getting better wages elsewhere that can buy their product. But if everyone works for megacorp, then everyone needs to be well paid to afford megacorp products and be happy consumers or you get unhappy employees and no sales.
Not to say that the CEO won't be ungodly rich of course, but its just that, when you control the entire workforce, and that workforce are also your customers, why would you treat them like shit? A happy employee is a happy consumer, and vice versa.
Anyways, remember there is always taxes and retail markup. Maybe you lose money because people are buying alien scarf communicators instead of iphones or samsungs, but you charge a 20% tarrif on the goods and you sell them at stores you own. Guess what? Turns out its a lot more expensive to buy an alien scarf communicator in your iphone/samsung phone than it is to buy your iphone/samsung. Some people will still buy them though, because they are new and different, thats ok, you make profit anyways. Dont worry, megacorps will ALWAYS find a way to make profits, even on inported products made by aliens.