Yes, if the domain is fully hosted by DreamHost, then this directory should exist in your home directory.
You can check if the domain is 'fully hosted' by looking in Domains -> Manage Domains
No, each domain that is fully hosted will have a directory of the same name in the home directory of the user who controls that domain. This system allows you to host many domains (or sub-domains) under the one user account if required.
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