Here we show that the Scale-Symmetric Theory (SST) is the realistic/unique String Theory realized by Nature. The SST starts from 7 parameters only (there do not appear free parameters) and a few very simple symmetries that follow from initial parameters. We present the realistic interpretation of the different string theories, the compactification of the higher degrees of freedom, new fermion-boson supersymmetry, new SS-duality concerning shapes (a self-similar duality), the S-duality (a strong-weak duality), T-duality (duality of different spacetime geometries; duality of large and small distance scales), U-duality (unified duality; a symmetry for combined S-duality and T-duality), the origin of radions and dilatons. There appear following fundamental spacetime objects: the luminal gravitating 26-degrees-of-freedom Einstein-spacetime components, the superluminal 10-degrees-of-freedom spin-1 entanglons responsible for the quantum entanglement and the 6-degrees-of-freedom non-gravitating tachyons carrying infinitesimal imaginary spin (existence of such tachyons results from extended General Relativity; the gravitational fields consist of them). There as well appear the 58- and 122-degrees-of-freedom stable objects composed of the 26-degrees-of-freedom spacetime objects. The 11-dimensional supersymmetric M-theory and its 10-dimensional components cannot be realized by Nature (it follows from the thermodynamics of the tachyonic liquid at the beginning of the inflation). New symmetries of the initial liquid-like spacetime lead to the succeeding phase transitions of the non-gravitating Higgs field composed of tachyons.