Subdoman and Sub Directory Sites

If you want to use subdomains and subdirectories in the same installation of WordPress, you are going to need to use a plugin to set that up. I believe you could use one of the domain mapping plugins to map a subdomain to a new site you set up in a subdirectory; or you could use one of the multi-network plugins (https://www.ads-software.com/extend/plugins/wp-multi-network/ https://www.ads-software.com/extend/plugins/networks-for-wordpress/ or https://wpebooks.com/networks/ ) to set up multiple networks within the same WordPress installation.

I believe you could use one of the domain mapping plugins to map a subdomain to a new site you set up in a subdirectory

that woudl be the easiest, yes.