I store my entire Plex library on an old Dell t420 server which has an old spinning disk raid array and it performs well enough. And if you’re able to direct play the files they you don’t even need a strong CPU when hosting Plex, you can run it on a raspberry pi.

Yea absolutely, people have ran it off Raspberry Pis and external USB drives lol

Yes most people use HDDs for this because their speed doesn’t matter when they’re just serving up a single (or even a dozen) huge files at playback speeds. They’re slow for hosting your OS because of the quantity and speed of reads and writes but this isn’t an issue with movies, TV, or music.