New Here - How to Set Up

Want to utilize 1TB HD of music through Plex - through Roku via TV connected to AMP and speakers. Can Plex just exist on a tablet in this situation? AMP is not wireless. Thoughts? Many thanks in advance…

The Plex server needs to run on a supported device.
This could be a PC compatible “slate”-type device (although I only recommend this for “music”, but not “video” media types).
But there is no Plex server software for regular iOS or Android-based tablets. These devices can only run Plex clients (which you can use to pick and “sling” the media over to your Roku).

Start here: https://support.plex.tv/articles/200288286-what-is-plex/

Followed by: https://support.plex.tv/articles/200264746-quick-start-step-by-step-guides/

Understood. OK, and many thanks. So, an upper end Tablet running plex server. Connected to this tablet is my 1tb HD with my music. Then this tablet talks wi-fi to wireless roku - then plays my music through to my wired AMP/speakers?

Yes, this is possible.
But running around with a larger slate computer, with an external hard drive dangling from it is not only unwieldy, but also dangerous for the hard drive and the USB port.

Therefore I recommend you to rethink all of this and either find a different computer in your home network (preferably one which is running 24/7), or get a NAS device which is capable of running Plex server.
Then you can use a regular mobile phone or tablet as your Plex remote control.