A good place to read about all the apps that end in arr is their wiki called servarr
They provide setup instructions and descriptions as well as downloaders and scripts. They also each have a GitHub repository and webpage with screenshots and more flashy promo material so you can get a good idea of what they look like and how they work.
Essentially they all automatically go out and download any movies or shows you’ve told them to look out for, format the files and place them neatly in folders that your media manager like plex or jellyfin can then make available for you to watch on the client devices on phones, computers, smart tvs, chrome casts, Apple TVs, everything really