It seems they are deprecating their standalone code review tool Upsource in favor of the code review tool that's integrated into Space. Upsource is nice because it allows opening a review for a set of commits which aren't necessarily continuous. This way, multiple people can work on the same branch at the same time, and open a review for their respective commits afterwards. You don't need to wait for your coworker to…
I’m not familiar with Upsource, but what you describe sounds somewhat like the Gerrit review process. Each commit is reviewed individually, but sending multiple dependent commits for review is supported as well. It is technically possible for a coworker to build on top of your unsubmitted changes (by checking out your commits and adding more on top). However this process gets somewhat painful if the coworker updates…
I've heard great things about Gerrit, but it also seems like one of those "strongly opinionated, mailing-list based" review systems, and trying to lobby for such a thing in the GitHub/GitLab web based world is practically a non-starter in my experience