“But isn’t time relative depending on the observer and what is being observed?”
That’s a tricky question. Relativity assumes that there is no universal frame of reference, so there isn’t some outside place that we can measure distances (including time) based on and apply it to all the different frames that observers are seeing.
That being said, if you really start critically examining relativity, there is a strong implication in some of the results that there IS a universal frame of reference. So it may well be that there is a universal standard we could measure time based on, independent of any observers, and Einstein just made the wrong assumption.
It is all so confusing to me, hence I said I won't lose sleep thinking about it. After we began space flights, scientists tested the validity of Einstein's theories about time being relative depending on frames of reference. Two identical timepieces, one stayed on Earth while another was carried by astronauts on a space journey. After the astronauts landed, the two timepieces were compared and differed in elapsed time, the reality just as calculated by theory. Elapsed time was less for the fast-moving astronauts. So confusing!