There are a few positions within Congress that will get USSS protection and those are Speaker of the House, House Minority Leader, Senate President Pro Tempore, Senate Majority and Minority Leaders, and Whips in both chambers.

Every Member has a risk matrix score concerning their safety. Capitol Police covers most Members (who request it and have a need for it) for the most part. In some cases though it can be expanded and other Members can gain USSS protection at least for a brief time. For instance, before the 2017 Congressional Baseball shooting, Rep. Steve Scalise was the Majority Whip but only Capitol Hill Police were assigned duty to protect him (USSS was not in charge of protecting Congressional Leadership at the time) but once he was shot USSS took over protection duties for him and the other individual on the Republican Team who was critically injured.

Then Congress changed the law to protect these people with USSS since they were supposed to be the "experts" and all of these people in Leadership tend to be big names and crazy people would likely go after them over a regular Member...

EDIT: AOC only has Capitol Police Protect. She actually lives in a building that a ton of Dem Members live in including Dem Leadership so that building tends to have a police vehicle or two in front of it at all times.