Here is an extremely condensed summary of Einstein's book. The principle of relativity has three facets or aspects:
The Principle of Relativity in its Restricted Sense
The Special Theory of Relativity
The General Theory of Relativity
Einstein favorite example is a traveler in a train and a bystander on the embankment. The traveler looking out the window, sees the bystander and the ground moving. The bystander on the embankment sees the traveler and the train moving. Einstein concludes that motion is relative and explains that this is the principle of relativity in its restricted sense. Einstein then asserts that this is self-evident. The principle of relativity in its restricted sense ends up as the basis, or more precisely as a pillar of the relativity theory.
The #2 and #3 items above, the Special and the General Theories are much more elaborated. Einstein notes that the principle of relativity is much more comprehensive than in its restricted sense. The principle involves speed of light, reference systems, simultaneity, uniform motion, time, gravity and much more.
The fact of the matter is that the "comprehensive" principle (the whole of the 3 items above) relies on the "restricted principle". Einstein, but for a few definitions, starts his book describing the principle in its restricted sense. After having exposed the special theory, and on his way for the general theory he reminds us that to start with every motion must be considered only as a relative motion.
If you "landed" on this page, this site introduces an alternative to physics that makes sense! Namely the Gravimotion theory.
Gravimotion focuses at the foundation of physics that is physics basic principles.
Even though the 2 words Relativity/Reality are almost identical, Relativity has little to do with Reality.
In the Gravimotion theory the principle of relativity in its restricted sense is dissected! That principle is found to be a characteristic of our observation of motion rather than a characteristic of motion per se. As a consequence that principle is not honored in gravimotion. And since it is at the base of the whole Relativity theory, gravimotion does not deal with the Special theory let alone the General theory! The fact of the matter is that gravimotion dissents and departs greatly from Einstein relativity; gravimotion switched-to and evolves-within an entirely different imagination track!
In favor of gravimotion is the fact that anyone interested can comprehend it. Gravimotion makes sense; gravimotion is simple; gravimotion is coherent.
The principle of Relativity is a characteristic of our observation of motion and not of motion per se!