Relativity is always about relations between two frames of reference, as far as I can see that is. The relations can be described in length contractions and time dilations. It becomes very difficult discussing those relations as some 'real array' measuring defined 'time and length difference's' between two objects in space. 'Simultaneity' is the closest description we have to it, as I suspect, and that one will hurt your head

as it does mine.

You just need to think of it as us having A relative B. Both in a uniform motion although, when compared to C, finding 'real' differences in 'speed/velocity' relative each other.

A will define one distance, B should be able to define another, and C a third, relative any and all of them. The same goes for the time dilation as it seems a 'symmetry' of sorts to me. The only way I see to really agree on a same distance, and no time dilation (ignoring their gravitational mass), is when being 'at absolute rest' relative each other. And that is a really weird thing. What I mean is that all three of them should have their own interpretation of distance and time, relative the others, all of them using their local clock and ruler to define what is 'right'.

So, which one of them are right? All of them define different relations relative each one of the other. And photons, or just light, definitely 'experience' time, as we experience them. Otherwise there would be no calculations on the 'speed of light in a flat SpaceTime' from our side, measurable. To me, it seems all to be about relations, but none of them the exact same. But your local time, and ruler, is, always the same to you, and that is what you use measuring other 'frames'.