We've recently been collaborating on features by having one person do some part of it, and then passing it along to someone else to finish it up. I'm personally really enjoying this process since it's a great way for me to catch up on new code that's coming in. It's also interesting to me to see how one of us implements a solution to a problem or feature.
I don't always look at every line of code merged though (gasp!), and when I work on the next version of something that was built by someone else, I usually try on my own to figure out where the code lives and how it works.
Most of the time, I would have a seen a bit of the code when the pull request was made and I had the chance to look it over. When I don't, I end up going to GitHub, find the pull request, and see where the files are. Then I open up my editor and read the code for a while before getting my paws on it.
I'm sure everyone has their own workflow, or maybe your company provides some guidelines/rules for you? Do you often (try) to look at the code that's coming into the codebase, or are you usually working only in your own domain? Or maybe you ask the person who wrote it to explain it to you while pairing? Would love to hear how you catch up on new code!