Rare decays: A window on new physics

We report on the extrapolated measurements related to the flavour-changing-neutral-current(FCNC) $b\to s$ and $b\to d$ transitions with the Belle II data. The branching fraction(BF) and raw asymmetry measurements of the exclusive decay $\bar{B}\to X_q\gamma$, time-dependent $CP$ asymmetry in the transition $b\to s\gamma$, angular analysis of $B\to K^{\ast }ll$ and $B\to X_{s}ll$ are discussed. We also report on the searches for the decay $B\to h\nu \nu$. Most of these analyses are extrapolated with 5 and 50 ab${}^{-1}$ Belle II data.