  Monolayers of a prototypical cuprate high transition-temperature ($T_C$)
superconductor $Bi_2Sr_2CaCu_2O_{8+\delta}$ (Bi2212) was recently found to show
$T_C$ and other electronic properties similar to those of the bulk. The
robustness of superconductivity in an ideal two-dimensional (2D) system was an
intriguing fact that defied the Mermin-Wagner theorem. Here, we took advantage
of the high sensitivity of scanning SQUID susceptometry to image the phase
stiffness throughout the phase transition of Bi2212 in the 2D limit. We found
susceptibility oscillated with flux between diamagnetism and paramagnetism in a
Fraunhofer-like pattern up till $T_C$. The temperature and sample
size-dependence of the modulation period agreed well with our Coulomb gas
analogy of a finite 2D system based on Berezinskii-Kosterlitz-Thouless (BKT)
transition. In the multilayers, the susceptibility oscillation differed in a
small temperature regime below $T_C$ in consistent with a dimensional-crossover
led by interlayer coupling. Serving as strong evidence of BKT transition in the
bulk, there appeared a sharp superfluid density jump at zero-field and
paramagnetism at small fields just below $T_C$. These results unified the phase
transitions from the monolayer Bi2212 to the bulk as BKT transition with finite
interlayer coupling. This elucidating picture favored the pre-formed pairs
scenario for the underdoped cuprates regardless of lattice dimensionality.
