Yeah, I get that. Unless you either read about it, or have worked in a corporate environment with calendars that have events pop up on them without your direct intervention, it's not something that would be intuitive.
What's important is that nobody has access to your stuff. They just want you to think they do.
Scammers can send calendar invitations to anybody. Google helpfully adds them to your calendar automatically. And you often don't get the invitation in email, since it gets added directly to your calendar.
Accessibility is about providing equivalent experiences. Presentation absolutely matters because bold, italics, etc. are used to indicate context. Markup exists to indicate context, not the other way around.
How familiar are you with ADA 508 and the latest WCAG standards?
Does it have to be on the Play Store?
I recently started using the Fossify Music Player. It's on F-Droid.