Different systems playing same music simultaneously

This is pretty easy with Chromecast groups. The Creation 5 control point also does it with diverse renderers, but Chromecast is much more reliable. 

I like that this was brought up . I’m sure many of us have a,b and c systems around the house .be nice to have set up when your busy around home going from space to space. Make leaving the sweet spot more tolerable. I’ve been meaning to pick ip a second one . Likely the new version so the wife can play apple stuff when I’m not around. Then and only then . 

I currently do this throughout my house across 4 different systems. One is my main home theater system, the other a vintage console stereo, the next a Fisher 400, and finally a set of computer speakers. I do it 2 different ways. The first is I have Google Chromecast Audio devices on each of them. They are no longer made, but, you can get them on eBay. They work great and use analog 3.5mm stereo jacks that you just put a 3.5-RCA splitter on. They’re wireless so zero cabling. To sync them up you simple create a “group” that includes all the devices you want synced up and name that group. Mine is called “the house”. The you choose whichever music streaming service you want as your default and simple say, “Hey Google, play Miles Davis in The House”. Then it streams in sync to each system wirelessly. In a separate input I do the exact same thing with Apple Airport Express’. They are also no longer made, but, again eBay. I do the same thing with those for my downloaded music on my computer. I just select all the devices from my MacBook in the audio drop down menu when Apple Music app is launched. Hope that helps a little.

An addendum to my post. You’ll need a Google Home or a Google Home Mini as a control unit to initiate the stream. Spotify, Google Music, Pandora, etc

Thank you for the suggestions. I see some interesting solutions...
What I have right now is a bunch of old style amplifiers. Some solid state, some tubes. No streamers, no modern receivers/amplifiers with network connectivity.
So I'd like to try something like a dongle that I can connect to each of the amplifiers to receive the signal (coming from a computer or a phone). I think there are already some receiver devices that can be synchronized in order to receive the same signal at the same time. Am I right?