it sounds like your manager is rather incompetent.

Obviously your boss is an idiot with poor communication skills. I’d quit and find a better job.

Either OP didn’t understand his full job responsibilities or the manager is incompetent and looking for someone to blame for the consequences of their inactions. My suspicion is the second - Incompetent manager. The most effective solution, assuming OP is not the incompetent one, is to look for a new job and then quit and let the idiot manager sort out what they should have done at the beginning and what they can do now. We all want to work where our boss is reasonable and competent. But unfortunately that’s not always the case. There is a fine line between asking too many “obvious” questions and winding up micromanaged and asking enough questions about the job, how things are done, and if there is a new plan. It probably would have been wise when these new people were hired to ask if boss expected you to train them or not, unless someone else was identified to train them at the time. Had that question been asked OP would have least understood what was expected. Unfortunately sometimes you have to carefully manage your boss - not manipulate them, manage how you communicate with them. Usually it’s wise to speak with your boss when things change in some major way so you are on the same page with your boss. A competent boss will do that, but unfortunately sometimes our boss isn’t as competent as they should be. You cannot change them, but you can reduce problems like this. Unfortunately, even if boss is clearly incompetent, don’t go behind their back up the management chain. Your boss interacts with their manager frequently and boss’s boss will tend to believe the person he usually deals with instead of you. And your boss can make your life miserable if they so choose. If directly asked a question by upper management be honest but careful, but only in the most egregious case should ever you go above your boss.

No one has ever told me I need to wear pants to work, either. Some things are so obvious, it would be insulting to have to mention them explicitly.