Snapshots – noob questions

[KC <impactoria@googlemail.com>]

I have been trying to understand how snapshots work (in BRTFS and in 
general), but I still have some questions, and would appreciate if 
someone could clear them for me.
To make things easier, I tried to make most of them Yes/No questions:

1. When creating the filesystem, I only made btrfs partition with no 
subvolumes. Do I have to use subvolumes, or can I snapshot entire “/“?

2. Can I take a snapshot of a working / root filesystem?

3. If I make a snapshot of / and there are some separate partitions 
mounted under /mnt/ or /home/, will snapshot skip them?

4. If I have a snapshot of  /, can I completely erase this partition and 
later restore it in full form that snapshot, or do snapshots work only 
if a limited number of files has been changed?
If the former, then does it mean that snapshot size will be comparable 
to the original data size?

5. Can a snapshot be stored on a different partition? And if so, does 
that partition have to be BTRFS too? I ask, because I would also want to 
store snapshots in a cloud storage.

Thanks.