Unallocated Disk Space can be assigned its own volume letter or used to extend another drive. Types of Partitions are also explained.

If Mark Partition as Active, Change Drive Letter, Format, Extend, Shrink, Delete Volume options are greyed out in Disk Management, see this post.

Not sure why they make you create the new partition using Disk Management there. It's just two commands in diskpart, which you're already running. But yeah I forgot there was a difference, though ...

NTFS is the primary file system for Windows 11 and other modern versions of Windows that supports disk quotas and large volume sizes ...