I’ll put some words around this at some point – but here it is as simple as possible.

gparted extend /dev/sda2

fdisk /dev/sda

create new logical partition
set type to 8e

pvcreate /dev/sda6

vgextend ubuntu /dev/sda6

vgdisplay – get free PE

lvextend -L20GB /dev/ubuntu/root

fsck -f /dev/ubuntu/root

resize2fs /dev/ubuntu/root