Android devices have to include at least some of Google's own applications if they want access to the Play Store (which is an issue on its own), so it's not too surprising when one of the company's apps reaches a significant number of installs. The management app for Google One is the latest application to reach 100 million installs, placing it in the same range as Google Classroom and YouTube Studio.
Google One is the company's paid subscription that includes more cloud storage, direct customer support, credits when buying from the Google Store, and extremely lame benefits. The app is primarily only used for subscription management and customer support, though it recently added a VPN feature.
It's worth noting that the Google One app is pre-installed on some devices, which inflates the download count a bit. Still, the number is a good indicator that Google One is enjoying some success.