It is completely normal to need to review information that you don't have at your fingertips. Even if you have taken and done well on courses in electromagnetism, doing a PhD in a subfield of electromagnetism means that you are now going to hyperfocus on a specific area, and you are very unlikely to remember all the details you learned about that area in your course. AndA good researcher is not someone who knows all the details they need and never needs to look something up; a good researcher is someone who knows how they can find what they need, therelearn it, and put this knowledge to use. There are some advantages to reviewing undergraduate lectures: (a) they tend to be at a higher level, which is useful at the beginning of a project when you are getting your feet wet with the subject and don't want to get lost in the weeds, and (b) more undergraduate lectures tend to be available online than graduate ones.
Additionally, while undergraduate lectures have the advantages stated above, at some point you will likely find that they do not cover the material in sufficient depth for what you need to do. Therefore, it is also important to look at other sources. Probably, you will find that you will need to read advanced books and research papers to learn the techniques you need. This can absolutely feel scary and intimidating if you have never used these kinds of resources before. And if you are experiencing feelings of imposter syndrome, then it's normal to feel like "you aren't smart enough" to tackle these more advanced resources. However, you don't need to believe or act on these feelings. You want to make sure that you are not using the undergraduate lectures as a crutch because they feel comfortable, but instead are making rational decisions about using the source(s) that contain the information you need at the right level to do your work. (I'm Your advisor can help you determine what the relevant sources are.
I'm not saying this is what is happening in your case, but ashere a common example of the kind of thing I am saying: some good. Many students, when they transition from undergrad who did very well as undergraduates have a difficult time transitioning to a PhD,. They find that theyresearch challenging given the difficult, open-ended problems and long timescales of a research project, and miss coursework where they did well and find research difficultgot rapid, andpositive feedback. So, they try to recreate the experience of taking courses. These students eventually have to learn to leave behind the successthat research progress is measured using different metrics ofthan coursework, and learn howchange the way they work to make measurable progress onbe successful in research.) Your advisor can help you determine what the relevant sources are.
Given that you were admitted to a PhD program, I absolutely believe you have the background needed to make the jump to using the research literature successfully. What you are feeling is very common, and not easy. It may feel like you are stepping outside your comfort zone at first, but the rewards for "learning how to learn" are enormous and will pay dividends throughout your career.