Does an Excluded Directory in Windows 10 Defender Also Include the Sub Directories?

Yes, excluding a directory in Defender will also exclude all files and subdirectories recursively.

Yes, subdirectories are covered by exclusions. HOWEVER, exclusions only apply to scanning, not to Real-Time Protection. If Real-Time Protection is active, every loaded executable is scanned, even those loaded from an excluded directory.

sub directories are excluded, too. documented here: https://support.microsoft.com/en-sg/help/4028485/windows-10-add-an-exclusion-to-windows-defender-antivirus