What makes everything in Einstein's theory relative

I get that everything is relative, but what make it true?

Why is everything relative? What is the true source of his theory that makes everything relative?

The constancy of the speed of light just happened to be what Einstein used to derive the equations for how time and space are changed for reference frames in relative motion. The true meaning of relativity is that ALL of the laws of of physics should look the same in ALL reference frames. That means no true constant of nature can look different in different reference frames, and there is no physical experiment that can be performed that will enable anyone to determine any absolute motion.

The key understanding, and this extends to General relativity as well, is that the interval between two points in space-time, will be agreed on by all frames of reference. Different reference frames will disagree on how much of the interval is space and how much of it is time so those are considered relative quantities, whereas the space-time interval IS agreed upon and is considered invariant (doesn't change) in different reference frames.

So, basically relativity tells us that we don't always see the 'whole thing' in space-time (e.g. the space-time interval). Some of the pieces (e.g., space by itself and time by itself) that we do see are just parts of a bigger whole and don't really have an objective meaning - just a relative meaning. If we all could see the 'whole thing' in space-time, we would see that as being real and everybody would agree on it, even though we would disagree on what the pieces looked like. After Einstein, physicists moved all of their equations into Einstein's space-time and it became very obvious what was objective reality and what was not objective (relative)

Einstein said that everything is not relative. He said that the speed of light is constant from the viewpoint of any observer.