If you want to use the REST API, here’s some info that might help, from a current discussion about the new Google Assistant 2.0 integration. Personally, I don’t recommend going down this path.
Note that the integration is moving away from tags and toward using metadata (but tags will still work for the foreseeable future). You can see the metadata definitions that Google will recognize in the documentation, but it’s all shown relative to item files, since that’s what the majority of people use.
My advice would be to take a step back from Google Assistant and first get comfortable with .items files. That’s partly for your own education in OH, and partly because it’s much easier for the community to provide assistance if you’re going down the same path that most of us are. Once you’ve got your devices defined in .items files, adding the Google Assistant metadata is a very simple task.
PaperUI is not in active development; it’s used mostly to configure the base system, add bindings, and discover devices. There’s work being done on a replacement, but that’s in the distant future.
It sounds like you’ve used PaperUI very effectively to get to this point, and you’re ready to take the next step. That might sound daunting, but with .items files there’s no significant coding involved: each device you want to control will be one line of text in a file. I would suggest that some of the things you’ve already learned are just as challenging, so I’m pretty confident that you can handle it.
When I get stumped, I start by searching the documentation and community, because someone else has probably had the exact same issue. If you can’t find what you’re looking for, feel free to ask! No one here minds helping out so long as we see that an effort has been made. 