Team Rocket battles are live in Go.
Look for a stop that is darker than others. As you approach it will get darker and have an R hovering above it. When you are in range of the stop a member of team rocket will be standing beside it like a pokemon. When you top of them it will initiate a battle similar to player verse player battles, but have rewards like raids.
The reward pokemon you catch will be a shadow pokemon. It will have a shadow around it as well as red eyes. You can purify it to turn it into a regular pokemon. Purifying it will increase each IV by up to 2 places.
Most people don't want to purify the shadow pokemon though, they want to keep them as is be cause they look cool. Also shiny shadow pokemon exist. So people are already saying they want a lucky shiny shadow pokemon. lol
So far the shadow pokemon I have seen have been the first 3 starters and a Zubat.