Of course you must have a local network to which all RasPis are connected. WLAN is OK for showing pictures and stream audio files, but for video streaming it is limited, so you may consider to use wired connections for higher resolutions.
As already noted in the question you have three components:
- the media server should run on a recent Raspberry Pi. It only has to stream the media from its storage so focus should be on a performant network connection. You can use minidlna that is available on the default Raspberry Pi OS repository and explicitly supporting armhf for Raspberry Pi. To use it look at Howto install UPnP/DLNA media server.
- the media renderer will get the media from the server and output it to its sinks, like connected speakers. The first step to setup it is, that you have to ensure it is playing the media as expected. For example it must play music to its speakers. For my power audio in the living room the RasPi has a high quality JustBoom-DAC HAT, connected to a 150 W amplifier with big speaker boxes (the neigbors have already rung the door bell :-), the RasPi in the bathroom just uses the built-in audio jack. If this is working then you can install gmediarender, also available from the default repository. it will receive the media from the media server and output it to the configured sinks. Look at Howto install UPnP/DLNA media renderer.
- the control point is used to control all of it. I haven't looked if one is available for a Raspberry Pi. I want to have it in my pocket, so only an app for my smartphone is possible. You will find many apps that can connect to a media server and use a media renderer. For example BubbleUPNP is one of them. But its not their main focus and its mostly not very stable and they have a lot of other things I don't need. For Android smartphones I use and recommend DroidUPnP. It's just a control point without other overhead which makes it lean and stable. Search for
droidupnp 2.4.1 in the internet browser of your Android smartphone and download it from one of the sources. I downloaded it from
APK-tools. When sitting on my Linux PC I use Upplay. I don't have an iphone so I cannot help to find a control point for it. Try one of the offered UPnP/DLNA apps.
These are all pretty simple, independent and lean applications that make them less error prone. The power of the environment is, that they work together on an open standard UPnP/DLNA that makes it flexible and easy scalable. No problem to integrate the smart TV. It is cool to walk from the living room to the kitchen, where you pull out your smartphone and switch off the TV in the living room, select your favorite song from the media server in the basement and play it on the media renderer in the kitchen.
Howto install UPnP/DLNA media server
Howto install UPnP/DLNA media renderer
Use Pi as DLNA server and serve media from NAS