Owning a store is not a monopoly. I can’t walk into walmart and start selling stuff on my own.
That's not what antitrust is about. Read about the Microsoft case and the Sherman Act. The store is just a tool.
If you want to focus on brick and mortar stores, imagine if half of the US was governed by Walmart, and half by Target. Imagine if you lived in the Walmart half and Walmart permitted no other stores in their states and added 30% to the cost of everything. Target did the same. Any competitors stores were banned by law.
Now the arguments that people are making are "well, you are free to move to the Target side of the country"
In the case of Apple and Google the market is the smartphone market.
I'm not sure if you remember the Microsoft vs DOJ case in 1998 - the arguments were all about what software Microsoft allowed PC manufacturers to ship on their platforms. At the time Microsoft had terms and conditions that prohibited any manufacturer that wanted to ship Windows from shipping anything competitive. They lost big time.
Apple has even more control than that today.