CNA and EMT are both great roles to get relevant experience.
CNAs definitely get their foot in the door for hospital work because in addition to getting really good at the patient care aspect, you learn a lot about the flow of the floor, see the effect of interventions, and are often the first one to notice a problem with a patient. Obviously, you would bring it to the RN's attention rather than handling it yourself, but it can give good clinical observation practice. It also gives the nurses and managers a chance to know you and be impressed by your work ethic. In my class, the ones who had jobs first were the ones who already worked in a CNA/PCT role, and were hired onto their units.
EMT is a bit different because you're not right inside the hospital, so you don't get to know the nurse managers, etc. But you do get to learn a lot of skills that are really helpful for nurses - IV insertion, EKG interpretation, administering a limited group of medications. Coupled with on-the-scene assessment skills, many EMTs find themselves in a strong position to transition to nursing, especially Emergency nursing.
Most volunteer positions are not worth it, if your goal is clinical experience or getting your foot in the door with an organization. Most facilities will not allow you to do any patient care at all. Before COVID, our volunteers did things like bake cookies or push around the comfort cart (books, magazines, lip balm, etc.). Since COVID, we don't allow volunteers on the floor at all. The only exception I can see would be hospice volunteer. While it might not directly translate the way CNA or EMT would, some home hospice volunteers do get to provide some direct patient care, and can get a really solid understanding of the dying/comfort process and navigating family dynamics that can come in handy later. This is probably not as helpful to your particular goals as CNA or EMT, but it's an often-overlooked opportunity so I thought I'd mention it.