I meet with my supervisor nearly every week
This sounds like a good level of engagement. Some people may prefer more frequent contact, some may prefer less (and it may vary by stage of a project), but I'd say "nearly every week" is a pretty solid middle ground.
Whenever I ask the supervisor on things I don’t understand they never have an answer and say they don’t know
It's reasonable to expect students to do their own work towards finding answers, and when you get into "research-level" investigation rather than "coursework-level", experts in the field do not have full knowledge of everything. There's too much "stuff" out there, it's not possible for even experts to be on top of everything except their very narrow world. Even most things they may have read may not come to memory on cue. This can even include details of ones own past work! Knowledge at the research level mostly involves being able to find the information you want efficiently and to understand it - sometimes this involves just knowing the general area enough to know search terms to use, sometimes it involves knowing particular individuals who may have written on the topic, sometimes it involves realizing you've read something similar before and finding the paper in some way you've organized papers (perhaps in a literal file cabinet, or in these digital days perhaps a PDF on your file system).
It's also quite common that there is no answer, or the answer depends on so many different factors that the answer is ultimately up to informed opinion, and different people will come to different conclusions.
Without knowing what sorts of things you are asking about, it's hard to know if your supervisor is just nudging you to do more background work on your own, or if they're pushing you off to save their own time investment, or if they're entirely clueless.
“I didn’t read it in detail but I thought it was good”
This response would be frustrating to me, too, but without knowing your advisor it's tough to judge. Perhaps they really do think your work is just fine. That's great, and nothing you need to do about it! Perhaps they think your work is good enough for a masters thesis. That's great, though unfortunate if you're trying to polish some skills, but should get you through the proximate goal of graduation.
Also I asked them about a key idea in the project that was central to one of the papers and they just said “I don’t understand it either”.
It's not entirely clear but I assume you mean this is a paper that is part of a project you're working on, with your advisor as one of the authors? It's unfortunate, then, if they are an author on a paper they don't entirely understand, but sometimes that's what happens when you collaborate. Hopefully they at least understood at the time of publication, but in this situation I might ask that they direct you to someone who does understand better. Sometimes experts rely on other experts with differing expertise: this is a much more efficient way to work in interdisciplinary areas than attempting to become an expert in everything (which is futile).
I submitted my most recent draft and after cancelling our meeting they said they would give me an email of written feedback and it never arrived.
This is rude, though it's reasonable for you to give them another nudge and ask when you can expert feedback. Hopefully they respond productively.
I don’t know if I’m expecting too much from a supervisor or if this one is the norm. I thought they were meant to be experts in the field.
Nothing in the story you tell sounds to me like it conflicts with your supervisor being an expert in the field. It sounds like there are some orthogonal issues: maybe they're too busy (which is a bad excuse, but not particularly uncommon), maybe they see your project as not particularly important, maybe they have more expectations of independence from the students they advise. What you describe sounds "normal" to me, but "normal" doesn't mean "acceptable". My own research advisors were always more helpful than what you describe. From the selection of questions we get on this site, others are much worse.
If you were earlier in the process, I'd suggest finding a different supervisor that aligns better with the guidance you're looking for. Being just a month from your due date, though, I'd keep a shorter view and look to just your immediate next goals. Ask very specific questions of your advisor about your next goals, not vague requests for feedback. Questions like: "is this work sufficient for a masters thesis so that I can graduate/get good marks/publish in a peer reviewed journal?" And, if not, what specifically do you need to do to fix that.