Can I access the content connected a rasplex at my phone and send to chromecast?
Rasplex is a client of a Plex server just like Plex is a client to your Plex server. chrome cast is a device that allows you to cast your media stream to. So if you have Plex on your phone you can cast to a chrome cast the Rasplex wouldn’t really need this functionality because in general you have your pi connected to a screen/projector/tv and the screen size it the desired size.
I hope this answers your question. Rasplex is not the server, Plex is the server. Rasplex is the client (just like your phone)