The default (legacy) setup on one drive is two partitions: one for /boot and another for LVM volumes.
The default (UEFI) setup on one drive is three partitions: /boot, /boot/efi, and third for LVM volumes.
The Debian guide was aiming for two RAID1 arrays: one for /boot and another for LVM.
One partition from each drive is used for array, so each drive has two partitions.
The /boot filesystem is in RAID1 array because then each drive have a copy of it. If the only drive with /boot would break, then system cannot boot even if the rest of data is still intact.
The RAID1 array that does host the /boot must use RAID metadata version 0.90.
One can have swap outside of the LVM, but that just makes the setup more complex.
These days I do use “Minimal”, because I do have configuration management system set up that install package selections, but “Server” sounds like a decent start. One can always add or remove packages later.
I don’t use security profiles.