iCloud Backup, on by default, is not end to end encrypted and uploads almost the entire contents of your device to Apple’s servers every night in a format they can read (effectively unencrypted). This includes your entire message history over SMS and iMessage and all attachments thereto.
It is safe to assume that the data collected and assembled by these daemons is part of the iCloud Backup, which you never have to explicitly enable; it happens when you log in to iCloud on the device, usually within minutes of first powering it on for most people.
The word "backup" does not appear at all in that webpage? Your claim is unsubstantiated. For these features to work, they must be running on iOS as well, for people who don't own a mac. Siri is used more on mobile devices than on desktops, generally.
iCloud is not on by default with macOS. For starters, you have to create an iCloud account!
It's an eternal balancing act because every time Apple implements some new restriction in their OS that prevents apps from doing whatever they want, whenever they want, people get mad at them. But without those restrictions, the ever increasing amount of personal data that is stored on your computer is exposed to the ever increasing number of data slurping applications that you run.
That not being the case is the whole purpose of the per-app sandboxes which macOS has along with TCC. We're not discussing plain Unix permissions alone.
I assume you mean 'many' and not 'make'—this is untrue; you only need to grant FDA if you're trying to access protected areas, like the knowledge graph store.
Granting full disk access to an app that you don't trust not to surreptitiously exfiltrate your data is the kind of user error where the buck stops with PEBCAK.