The US intelligence community doesn't have backdoors into all phones. They have backdoors into the phone carriers for certain, though. AT&T, etc have fiber optic runs to spy closets where audio is recorded and speech-to-text tools are used to help search for key words. Snowden wasn't even the first to know about it. I remember when Shia Labeouf talked about it during an interview where he worked with the feds to prepare for a movie. He mentioned government spying, and the feds played him back a recording of a cell phone call he'd made years before to show off what they could do. This is why everyone who actually works for the government in high positions all use encryption for their calls and texts (while simultaneously fighting for backdoors for encryption) -- because those apps circumvent the carrier's recording technology.
Phones are pretty rock solid - especially Apple iphones. I've heard from local law enforcement that there's a huge backlog of iphones and other equipment with strong encryption that the feds can't break into yet. For simple codes on Android, they use a USB that fakes a keyboard input trying all the possibilities 'til it unlocks - doesn't take long for simple numeric codes. iPhones will make you wait between tries and the wait time gets longer with each failed try. Biometric ones are easy to unlock - use a lifted print or a photo if it's a face unlock... but, they still have to unlock it b/c there's no actual backdoor.
Verizon and some other carriers have their own OS modifications for Android, so who knows what they put on their phones when they flash the ROMs to make them work. I assume carrier unlocked factory-default phones would be free of such spyware, but simply making a call on a carrier means the carrier can listen in to the call since they make the connection.
Any funny looking hardware would get scrutinized and would kill a phone maker's business if found, but software can be tricky. Apple is the only company I know of for certain that loads their own un-modified OS on their hardware. Verizon, Sprint, and others tend to tweak the OS and flash the ROM... they could be doing anything, really. Huawei, if it were allowed to play in the US market, would be subject to Verizon, Sprint, AT&T etc... the carriers would mandate what software was on the devices and flash their spyware if there is some there to flash. Huawei could have a super-secret hardware firmware backdoor, kill switch, or the like, but a physical rogue chip would be detected, and any malware would have to navigate through the flashed telcom firmware to operate. The minute malware is discovered, their business would be over. It'd be suicide for them to do that.
The US is just upset that China isn't following US sanctions on other countries in addition to the current trade war. Huawei being Chinese government controlled will always be a threat to US security as at any time, Hauwei could flip a switch, flash an update, and own your device.... but, it wasn't until recently that the US told their govt contractors to ban Hauwei devices. It's all politics at this point.