Encryption disk

I have thoughts to encrypt my disk.

I found article Ubuntu Desktop 20.04: installation guide with btrfs-luks full disk encryption including /boot and auto-apt snapshots with Timeshift.

And I don't understand one moment.

In article author use partman:

/usr/lib/partman/mount.d/70btrfs/usr/lib/partman/fstab.d/btrfs

instead

/etc/fstab

and put in it some mountpoint lines.

Questions:

1. What is the difference between partman and fstab?
2. What do realy partman do and maybe it is really better then fstab?