I've been the social media person (community manager) for a few large companies: HTC for a few years, a Google account for a bit, then more recently for Qualcomm. At least for the larger companies in general that I've worked with, "interns" never touched post copy. Outbound "posts" are typically written by a group of people outsourced to an ad agency, approved internally, then approved by the client, then scheduled to be posted. Typically dozens of people are involved for a single tweet.
Companies (at least the ones I've worked with) take their social media handles as "official" messaging from the company that could easily effect stock prices if incorrect messaging is shared. I once had a colleague incorrectly share that a budget smartphone was going to receive the latest version of Android (at the time) and several tech news sites picked up on it.
Direct replies to users is a bit different. Typically a response is drafted in a tool like Sprinklr then approved by a second person.