Roku is like an operating system, like a simpler version of android.
Roku apps are called channels, there are lots of official channels and even some unofficial channels (beta and such).
there are some paid roku channels, like games and screen savers and themes etc, but pretty much all the standard streaming apps you just add the applicable channel and login with whatever your login is for that service.
There are also channels which have ‘free tv’ and movies, but are mostly full of ads etc.
So to answer the question, for roku there is a plex channel (and a plexpass beta channel), you log in with the 4 digit code and then you have access to any servers/content your account has access to.
As far as full hd, it is not necessarily roku or plex that determines what can be played, it is the TV and whatever audio/video codecs it can directly back.
My tcl roku tv can play back 4k hdr from my plex server. It can direct play x264/x265 content, but will only direct play lossy dolby digital and DTS.
Lossless audio as explained previously will require e-arc which is forthcoming with HDMI 2.1 supported tvs and receivers.
Roku and/or plex directly on the TV is convenient for simple uses and users, but if you want the full audio experience (without hdmi 2.1/e-arc) with atmos and other lossless audio, you will need a 4k plex client that goes into your 4k atmos receiver.
The nvidia shield is not perfect, but it does work and will get you 4k hdr and atmos/etc audio, if you have the appropriate receiver and tv.