Now mind you, everything I write might be wrong, I am out of my depth here.
But as I understand a BTRFS snapshot is simply a (subvolume in which you will find) copy of the table that points to the actual files or, rather, blocks on your drive. As long as a table exists that points to a block, this block will persist.
The nature of BTRFS is Copy-on-Write so in your active snapshot, when you modify a file / block, a copy of it is created with the new version, referencing this new block on the filesystem table.
This is why BTRFS snapshots are fast and take little space by themselves, you do not need to actually copy all the data at the moment of creating the backup, rather when the data is modified and only that data.