And for those of us without a PHD in network engineering
OK, I shall try and articulate...
If you were streaming a multiplayer game from Google's DC the hard work of processing the game is done there so all they have to do is ensure they have the capacity in the Servers and Network (think free lanes on a motorway) to send that traffic to you.
Once it leaves their Network it will go over intermediary networks to the ISP/Carrier Network of the Stadia Subscriber where the ISP will route the data traffic/packets correctly to you like Royal Mail deliver your mail or courier delivers your OcUK order
.
Once at your door via the Router, its unpacked by the relevant device hardware/software e.g Console/PC/Stadia for you to see on your display and interact with via your controller/kbm.
In theory, it works exactly the same in reverse (bar the only thing that's being sent is your characters inputs to update the server/host of the game). The only real difference is you are doing the game processing at your home on the console/PC, therefore in multiplayer games as you are reliant on the same paths/steps taken above working correctly (just in reverse) with little enough delay or latency to ensure it runs smoothly without lag.
TL;DR
The only real world difference to consumers is where the game processing is done hence the whole 'cloud gaming' SP.
Shawrey