Plex is a logical term for describing where your volumes are located, if you are not using SyncMirror then you have just one plex, otherwise you have plex0 (with actual data) & plex1 (with a mirror of a data).
(credits ::radek kubka)
Also, if a volume is mirrored does it mean that copy of the volume resides on plex1 and original one resides on plex0.
Answer is Yes
Refer this doc for the complete info about syncmirror enabling and working