Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

It's a web service you can host on a server, connect to Radar, Sonarr and Plex and use it to find new shows and add them in a few clicks... Don't know what you're talking about there to be honest

You should think of Overseerr as a single install the same way you think of Plex. For instance, you don’t install Plex Media Server on every device you have, and then copy all your media to each device, right? Same principle applies here.

You want one Overseerr instance to live in one place (why not the machine you run Plex on?), then have everybody connect to THAT machine using their web browser. If you’re all on the same network it’s easy, though you might need to open up some ports on your firewall. If you want it to work over the internet, you’ve got a little more work to do.

you don't need to "repeat it on any device you need to use it on". it sounds like you set it up as a local instance, but the general idea is you'd make that instance available from any device on your network (or the whole internet if you enable port forwarding on your router)

as an example, i have it running alongside radar/sonar/plex on my media server and use NGINX reverse proxy to make it available anywhere from https://requests.mydomain.cat/. "nginx proxy manager" can help get that domain set up securely with your own auth rules

So I don't use overseerr but I do use jellyseerr with my emby setup. Both are just webservers that use a webpage/site that you can login to and add/search for content that will then be sent to your -arr stack and finally indexed into Plex/Emby. You can install overseerr on whatever machine you like, but to access it your other devices need to know where to find it (ie IP:port of hosting machine). You should really set up overseerr on a machine that runs continuously, so like others have said, likely the machine you run Plex on.

I personally take this a step further and use an internal custom domain name (ie. jellyseerr.mymedia.com) that can be accessed from any device on my internal network. I set my router to capture all domain requests for "mymedia.com" and redirect them to a reverse proxy (swag in my case) that will then forward the requests to different IP:port combinations based on subdomain. For example: emby.mymedia.com, jellyseerr.mymedia.com, radarr.mymedia.com, etc. This allows you to access all your services using easy to remember domain names instead of IP addresses.

The easy way IMO is just getting Overseer to see your Plex "watchlist". Then anything you want just search there and add it, Plex's Watchlist updates to Overseer and is added to the queue.

If you don't want to go down the path of opening up overseerr to the network and having to browse to it as others are suggesting (and is the normal way to use it), you could just set it up to watch the Plex watchlists and automatically add them that way

Then in Plex, you just search the movie or show you want, add it to your watchlist, And overseer will grab it and send it to radarr or sonarr to download

I don't recommend this method because it's not how overseerr was designed, and you miss out on a bunch of the features, I'm just offering this as an alternative since I'm guessing you aren't too familiar with web services on a network