By default the server logs will be under /domain/servers/<server_name>/log. The writable dirs (data, log, tmp) for a domain mode server will be children under /domain/servers/<server_name>.
In the host.xml 'servers' element you can set directory-grouping to 'by-type' (versus the default 'by-server') and then for each type of writable dir the servers will be under the same directory structure where the Host Controller process writes. So for logs it would be /domain/log/servers/<server_name>. But that's not the default setting.