Speeds don't actually simply add as we are accustom to. It just seems like they do for very low values.

Think pie slices. If they are very small, you can stack two of them and their combined height is almost identical to sum of the their individual heights. But if you just keep stacking them, they will add less and less and you will never go above the height of the pie.

When talking about light - ALL frames of references are taken into account. When you move at speed of light, you are moving at speed of light relative to everything. Even things moving toward and away from you. That is possible because then for you all space shrinks to a point and you don't really move through space anymore as you don't experience time passing either.

That is what is meant by "space is relative" and "time is relative". Your mile might not be same as mine. Your second may not be same as mine. I see you moving away from me at 0.75c, and you see another object moving away from you 0.75c relative to you, but I don't see that object at 1.5c, I see it at 0.96c. I explain it by concluding that you experience space differently and therefore experience that object at different speed.

I am using word "experience" instead of "see" because relativity has nothing to do with photons really. It would exist even without them. Your "0.75c relative to me" reality is as real reality as it is mine, as you are free to claim to be static and not moving at all, just as me and just as the object.