Each u3a will have its own instance of WordPress and associated database. This includes the core WordPress code, plugins and theme. This means that whatever is changed on your site, such as updating a plugin, will not affect other u3as.
The cloud hosting environment – computers and operating system – will be shared as they are now with Site Builder. The key difference is that Site Builder has one database and one suite of application programs shared by all the Site Builder sites. One benefit will be that a SiteWorks site will be independently backed up and individual sites can restored.
To learn more, do an Internet search for VPS (Virtual Private Server) or read this Wikipedia technical article en.wikipedia.org/wiki/Virtual_private_server.