Hello :wave:
Core Paypal's stack is potentially slightly different than the subsidiaries and acquired companies.
At Paypal, for the web-tier we completely moved to node.js in the past 3-4 years. In the client, we have quite some freedom to explore frameworks of our choice (Angular, React, ... even the old timer Backbone in a few cases).
For mobile native, the Paypal App uses swift on iOS and Java-Android. For one of our checkout SDKs (which is an effort I am leading currently), we have been exploring using react-native.
In the mid-tier we mostly write Java services. A few orchestration services use node.js as well. There are even a few legacy systems using C++.
For analytics, we use PIG for mining flat data alongside Hadoop and SQL scripts for db sourced data.
For a big company, we take pride in moving fast to explore newer technologies. At the same time, as a big company, it takes time to deprecate older stacks due to the humongous effort involved :))