Apple is interested in maintaining full control of what apps can be on their platform and how they are presented because it gives them power over negotiations with companies that build the apps. They are basically able to "name their price" and make sure they are always getting as big of a cut as they would like.

The EU is interested in not letting them do that because that kind of "negotiating" behavior is pretty well understood to be anti-consumer. Increased costs for app developers are usually passed directly onto the consumer through the prices. And it tends to get worse over time.

No company anywhere wants to use webapps anymore. Apps installed on devices are free advertising and access to user data. It is frustrating but the way it is, on all devices, already. So basically the answer is the same as why can't most apps that already exist on all devices anyways just be web apps.

I don't think sideloaded would be quite the right word, this is about access to other app stores (like the google play store or amazon app store, or more niche ones) that would then formally and automatically install and maintain apps exactly the same way the apple app store already does, presumably just with a different library of apps to choose from.

Apps from another app store would need no access to any API by apple unless they were specifically interacting with apple services, AFAIK. Which, would be under the full control of apple and apple chooses who uses it, how, and how much they use it, but that is already the case regardless.

I tried to answer your confusions as best as I can do with what I know already. As for why people take this so personally, I would say it is a complex topic combining businesses that are constantly trying to drive each other out of business with the social effects of making the tool people use to communicate a status symbol. And it has been brewing for long enough that people are getting extreme opinions and fostering long term grudges based on personal experience, to the point that some people have some real hatred towards anyone who has a different phone OS than them.

This was a long comment to type and I did it while laying in bed half asleep. Sorry if it has a bunch of typos or errors lol