If the future isn't fixed yet, how can we be in contact with our future selves?

I'm confused about the nature of time. On the one hand, I hear you and other people talking about how we have free will, and how Gee up on the GFL mothership is looking at the possible timelines and that they have certain probabilities to occur which keep changing. On the other hand, I'm hearing about people contacting their future selves and how they might even be guiding us, about beings from the past or future contacting us in the present, and that actually the flow of time is an illusion and everything is happening simultaneously. I'm also hearing about 4D beings and that the 4th dimension is the dimension of time, which I also struggle to wrap my head around. Is there any way you could explain the nature of time so that my scientific 3D mind can understand it?