Every quarter I write these things called OKRs (https://en.wikipedia.org/wiki/OKR) which is kind of like a plan for what you want to get done over the next 3 months. Often times if I'm unsure what to work on during the day, I'll read over my OKRs and see what needs attention.
These days I'm doing a lot of work :focus-visible (https://github.com/WICG/focus-visible) so I'll probably fight with some Sauce Labs tests or review PRs. One of the really interesting aspects of incubating a standard in the WICG (https://www.w3.org/blog/2015/07/wicg/) is that it's all open on GitHub (no mailing lists) so folks show up with really interesting use cases or questions. We've evolved :focus-visible a lot over the last year based on that feedback.
I also attend meetings with product teams who are working on accessibility. This could mean discussing how to make a particular experience in an app accessible, or looking at what kind of behavior we'd recommend for a particular widget in the Material design system. I find this work super interesting. Usually these folks are already well versed in the topic of accessibility, and the problems they're confronting are just new and difficult.
As far as tools go, I use a pre-touchbar macbook pro (I'm too scared to upgrade), and a lot of time in Google Docs :) VS Code is my editor and I try to put all of my random thoughts down in Evernote.
To answer your last question, I think what Polymer really got from React was their model where ui = function(state). You see this now in Lit HTML. It's a really lovely way to build components. I think the React team did an amazing thing there. I remember when our brains were all pretty exclusively stuck in MVC land and they found a way to shift things a bit. Really cool stuff :D