Managers are only very tangentially involved with "rewarding" employees at Google. They do not promote them: it's the responsibility of the employee to apply for a promo, though manager has to support. And promotion decision is not made by the manager either: it's made by a committee that has never even seen the employee and knows nothing about their work other than what they choose to put in their "promotion packet", as well as peer feedback. IOW your manager can torpedo your promo chances, but they can't unilaterally "promote" or "reward" you per se, aside from some fractional bump in the yearly perf score.
So employees have to support the assertions they make in the promo packet with evidence. I suppose you could support the assertion that you've done great maintenance work on something that's valuable to the company, and show off the metrics, PRs (or "CL"s in Google terminology), monetary impact, and things of that nature. But Google culturally doesn't really give a shit about this kind of thing, even if it's otherwise impactful. A few people manage to do well maintaining code at scale, but by and large my advice to Nooglers would be to stay as far away as they can from anything that's not contributing to a launch in some _easily quantifiable and attributable_ way. Or even if it does contribute to a launch, stay away from things they personally can't take credit for. Otherwise you'll end up working on something that someone else "started", and that someone will take credit when it's done.