Explain stifle, or more specifically how Apple is doing that ?
Anybody (possibly except Epic Games) can develop and publish on the App Store. There's a cost associated with it, which in Apples case is 30% (or whatever you negotiate apparently). If you play by the rules, you can keep doing that as long as you like.
If you rent a shop in a shopping mall, there will be costs associated with that as well, and it's almost guaranteed to be more than 30%.
That is essentially what Apple is providing for those 30%, they provide a shopping mall where you can expose your goods, and people can pay for them. They handle the pesky stuff like refunds, (international) taxes, compliance with various government requirements, EU rules, and everything else. They even handle potential lawsuits for you (provided your app wasn't the one breaking laws).
They also vet (mostly automated) apps to ensure they're not using private APIs. That is for your protection. It's not an evil scheme by Apple to keep competitors out, it's for protecting the end user from bad actors like Meta scooping up all your personal data for "backup purposes" via some internal API.
Here in Europe we've had "alternative app stores" for a year or so, and despite living in a country where ~70% of the population uses iPhones, I don't know a single person that has ever used an alternative App Store, just like I don't actually know anybody that has downloaded an alternative keyboard despite those being available for a decade or more.
There is really very little you cannot do on the App Store in terms of features, so for many end users it is not a problem.
You may not like the price associated with it, which is what most of these complaints are about, the fact that Apple scoops up 30% of recurring subscriptions created through the App Store as well. People tend to forget that running your own infrastructure is also not free, especially when you need to handle refunds, legal matters and international compliance.
And that's the core of the problem, most of these companies complaining wants to use Apples built in App Store tools, but they want to direct them to their own App Store for free, ditching the complicated stuff of dealing with users on Apple. They're more than happy with Apple to handle payments and refunds if they do it for free.
Sideloading is usually a very bad idea in this day and age. In northern europe at least, your phone is quickly becoming your most trusted device in matters concerning anything state or municipality, and here we have a national ID app on our phones, along with social security, healthcare, drivers license, micro payments, taxes, childcare, hell, there's even video conferencing with your GP, in an app that has access to your medical records, including bloodwork and various scans. There's literally no way in hell I'm trading the perfectly walled garden for the Wild West outside.
Anecdotally, where I live, most companies don't allow Android phones as company phones as they're considered insecure, and instead mandate iPhones. The more regulated the industry (medical, banking, power, etc), the more certain it is that you'll be getting an iPhone.