imposter syndrome and climbing the steep learning curve
I think everyone feels dumb at some point during their PhD, and if they say they haven't, they're lying to you.
One conversation I had near the end of my time in the lab was with a 3rd year PhD student who had just transferred into the lab next to mine. The toxic work environment in her previous lab pushed her to seek out a new advisor, and she landed in a lab completely different from her previous work - RNA work and molecular biology instead of neuroscience. She was frustrated with how much she didn't know, and intimidated by how much progress she felt she'd have to make to even start understanding her project. She felt like it was impossible to understand her new field in enough depth to progress her research.
I totally empathized with her. I'd spent the first two years feeling dumb, the next two being frustrated, and the last two owning the fact that I felt dumb. The thing is, that's actually a very common experience for PhD students. Ask anyone who's finished their PhD and they'll tell you that the difference between junior and senior PhD candidates is going from "I don't know anything compared to everyone else and I feel sad" to "I don't know much but that's OK". Literally the sad vs happy guy on the bus meme.
Here's the thing. You're going to start off in your cohort with fellow high-achievers (because who else would apply to and get into a PhD program?). You might be in classes with them where everyone else seems to know the answers and you don't. People could be throwing around terms for techniques that you don't know. I mean, I came into my program never having done a Western blot or designed experiments (my technician experience was just generating NGS libraries). I'd sit in these classes with everyone else that were meant to introduce you to scientific thinking under the guise of learning a broad range of biology topics, and when our professor would ask the class to propose an experiment to answer a question, I was stumped while my fellow classmates would know the answer was to use liposomes. I didn't know what siRNAs were or that you could make overexpression cell lines, and I didn't really remember the different functions of organelles.
What I said to that 3rd year student is that IT'S OKAY. It's normal and fine to not know. You aren't an imposter. You are in graduate school to learn and train. Maybe some other people have a leg up on you, but that doesn't mean you can't learn. I can't tell you how many times I googled terms in papers I was reading, or decided to be brave and asked what I thought were stupid questions to trusted peers or mentor figures. Trust me - someone who is humble and willing to admit when they don't know something is a much more trustworthy scientist than someone who makes stuff up to answer questions about their research talk. Being inquisitive and curious and approaching everything as a learning opportunity rather than an obstacle will do you a lot of good.
What about knowing your field? Well, the entire 5-6 year process is additive. Maybe it's easy for me to say being on the other side of it. In my first few years, I was blown away hearing senior students talk about their work and seemingly know factoids off the tops of their heads that fed into their logic when answering questions. Meanwhile, I felt dumb constantly reading the same passage in a paper over and over to try and understand it and browsing review after review. But it turns out that after weeks, months, years of reading the same general literature, that stuff sticks in your brain! (You also get to read lots of contradictory research.) I'd bet anything that hearing senior students spouting off specific knowledge is a result of constant exposure. They (we?) don't learn that stuff overnight, and neither will you. It's part of the growth process.
Trust me, you're never going to feel like you have all of the answers - I was still looking things up and learning new information while writing up my thesis in my last two months. Working on a thesis project means you're probably trying to answer a niche question that no one's answered before you, so you're bound to have limited knowledge. So go ahead and ask those questions, watch those Khan Academy videos, and read those reviews. You aren't an imposter for not knowing all of the answers; you're a scientist in training seeking those answers out!