Internally, all around. A 2018 answer by a Google engineer:

Build system automation of all kinds (although the main build tool is in Java)

Test automation

Deployment automation

Many services configure their job placement and options with a Python-derived language (not mine)

Monitoring configuration and dashboards

Data analysis using Scipy, Scikit, Numpy and various kinds of notebook UIs

Configuring Tensorflow and analysing the results of ML projects

Hundreds of little command-line utilities for all sorts of small code and data manipulations

Thousands of internal websites

The youtube.com site was still Python last time I looked

One thing I know of off the top of my head (because I had to deal with it last week) is the gcloud suite of tools for GCP, which will only run with 2.7. Thankfully the various GCP libraries for interacting with the platform do work on Python 3.