Special Relativity is about "flat" TimeSpace, it is NOT about constant velocities. General Relativity is about "curved" TimeSpace, it is NOT about acceleration. Acceleration is just the process of moving from one inertial frame of reference to another inertial frame of reference.

          Standard textbooks do not present the concepts of Relativity well. In particular these books askew understanding in favor of stressing calculational acumen blended with an excess of anthropomorphizing. I will try not to succumb to these same faults. So, when I write, "Bob and Alice perceive the events of the Twin Paradox via arrays of rods and clocks.", I need to carefully explain (in some detail) what that sentence means.

          Fraternal twins, Alice and Bob, travel to a, very large, mass free region of intergalactic space. This region of space is so isolated that no curvature of TimeSpace is measurable except for the incredibly tiny mutual gravitational attraction between Alice's ship and Bob's ship. The engines of Bob's ship will not be turned on for the duration of this exercise.
          Alice and Bob synchronize their identical clocks to read zero as Alice starts her ship's engines.