can family members share subscriptions

"Purchase Sharing doesn't include subscriptions. But if you use Family Sharing, you can share your Apple Music family subscription, and your iCloud storage and Apple News+ subscriptions, with your family members."

from: What types of content can I share with my family using purchase sharing? - Apple Support - What types of content can I share with my family? - Apple Support

In answer to your second post, you cannot selectively share subscriptions to individual members of a Family. In other words, if the second person has a family level subscription (and note it will often have to be a family level one, not an individual level) then it will get shared with the rest of the family. So in your case the 2TBs plans are shareable and if one person cancels the rest will still share the remaining one unless that person turns off sharing for the plan.

Note that the behavior of storage sharing can be a bit odd at times and is further explained in this document:

To find out 'What happens to your iCloud storage plan when you sign up for Apple One' click here --> What happens to your iCloud storage when you sign up for Apple One - Apple Support

The document describes various scenarios, including ones where you may have two subscriptions going at the same time.

The person who no longer wants to keep their iCloud storage subscription, can join the Apple One subscription.

They only need to cancel it if their subscription is more that the one offered by Apple One. If it's less or equal to the one from Apple One , it will be automatically cancelled after the Apple One free trial is over.

See here: link-> What happens to your iCloud storage when you sign up for Apple One - Apple Support

Don't cancel it manually unless its more than what Apple One is offering, and they have joined the one provided by Apple One already as data and file loss can occur when the plan goes back down to the free 5GB between changes.

Verify the total iCloud Storage is reflected as the sum of Apple One plus their own subscription, before cancelling there own subscription the normal way. Link-> Downgrade or cancel your iCloud+ plan - Apple Support

Yes, as long as the subscription is shareable.

Thinks like Apple TV+. Apple Music, iCloud, Apple Arcade etc... are shareable.

Third party subscriptions like HBO Max or Microsoft Office 365 may not be shareable at all, or may need to be shared outside of Apple's Family Sharing platform.

See the support document you posted from for details.