I found that the best way to learn about job openings was through the connections I made by going on informational interviews. When an assistant or editor position opens up, the first candidates considered are usually recommendations from staff or contacts of the hiring manager.
To make connections, I cold-emailed editors to see if they would meet me for a quick coffee while I was interning in the city or have a phone call when I was back at school in LA. I did my research, studied the mastheads of my favorite magazines, and sent a personalized email to each editor.
In each email, I told the editor that I was a student who wanted to pursue a career in the magazine industry. I listed my internship and other experience and concluded the email saying that I would love to hear about their career path and any advice they may have for me.
I received at least one response from every magazine I reached out to. However, if you don't hear back from anyone, don't be discouraged. Review your email closely for anything that could be putting people off, then keep reaching out. You will eventually connect with someone.