A number of years ago, Huawei was a small insignificant company which opened an office in the USA, they proceeded to start buying up Cisco Systems equipment, buying a few of everything Cisco sold, including the IOS and operating manuals.
They shipped all that equipment, software and documentation back to China, reverse engineered it and added to the equipment the ability for the Chinese to use it for spying purposes. They also attempted to essentially buy off Cisco Engineers to provided proprietary information on Cisco Technology.
This is one of the reasons the US was really pressuring our allies to not purchase anything from Huawei because it would compromise every installation where the equipment was installed.
Here is just one article where Cisco was suing Huawei.
If you don’t know who Cisco Systems is, they are essentially the Google of computer networking equipment, routers, switches, security devices, etc., everything from consumer products all the way up to carrier class equipment. If all the Cisco System equipment in the world shut off at the same time, the entire world’s economy would shut down immediately.
20 years ago Huawei was shipping copies of Cisco manuals with their equipment with the Cisco logo still on them.
Huawei didn't add the ability for the Chinese to use it for spying purposes, the spying capability was already built in under orders from the NSA before Huawei reverse engineered it.