The GreenscreenFootage group represents your footage. You pipe your footage into the deltakeyer.
Then, you need to merge your original footage back onto the keyed image where the deltakeyer either keyed out green parts that should be visible or parts that turn yellow. You do that by using a Merge and pipe a Mask for that area into the blue mask input of the Merge.
I'm not in front of Fusion right now but I believe I also uses a Mask on the white Solid input of the Deltakeyer. That's not necessary because you add the original image on top again, but I wanted to include that in the file because it's important to know.
Often you won't key like that, though. Usually you work with several keyers, combine alphas, and despill your image yourself. But I assume that would be a workflow that's a little too advanced for you yet.
Generally I would advise you to get a little more comfortable with the basic concepts of working in a node based compositor, as it seems like you've struggled with that.
There are some very good Fusion beginner tutorials from Blackmagic themselves in the Resolve 15 manual I believe.
And there is also a tutorial from me. Slightly more advanced, and nothing to do with greenscreen, but it covers A LOT of ground and will make you familiar with a lot of different parts in Fusion.
https://noahhaehnel.com/blog/create-an- ... i-resolve/