I don't know why people are so adamant it doesn't stop you getting covid. It absolutely can, in the way most of us understand that.
And in general, immunity is not taught in school, nor is how vaccines work, so I do think its useful general information most people don't have.
Simple not scientific version: if you have a viral illness, your body is attacked by it and to respond it makes antibodies as specific to the virus as it can to defend itself. Sometimes, your body can make enough to fight it off but not enough to stop you getting seriously ill, sometimes your body does an amazing job and you're not sick at all, sometimes if its a virus you've had before - say something like chicken pox - your body already has the antibodies floating around from the last time and essentially 'zaps' the first sign of the virus so its practically the same as not getting it at all. But in some people, this works better than others, and some people respond to some viruses better than others. And also some people have had previous viruses that while not the same exact one are close enough that some of their existing antibodies are able to start to fight it off while they work on producing specific ones.
Vaccines are artificial ways to get your body to produce the antibodies in advance, so then when you're exposed to the illness you can fight them off. Again, sometimes this mechanism just doesn't work in some people: their bodies don't really respond to the vaccine. In some people, their bodies respond, but not as strongly: they have antibodies, so they're not as sick as they would be if they caught it without the vaccine, but they don't have the level of vaccines to 'zap' the virus early on. They're still sick, just not hospital-level sick. In some people, the vaccine works perfectly and there's not sign they'd had any exposure to the virus. In all these groups, the artificially produced antibodies often don't last as long as the ones your body produces if you've had the disease, and for many viruses both the naturally-produced and vaccine-produced antibodies wear off in time. This is why boosters are needed for some vaccines, and why some vaccines they say last a lifetime.
I don't think you're stupid OP: and I get the point that initially people didn't know how effective it would be. We do now know people can still get it double-jabbed, how likely that is does depend on the specific strain, but across the board people are generally a lot less sick with it and way way fewer people are hospitalised/dying. So its a good thing, and it works in line with how it was expected it would work but nobody really breaks that down in detail.