Rare $b$-hadron decays serve as powerful probes for investigating Lepton Flavour Universality (LFU) and Lepton flavour violation. These decays, mediated through $b\to s\ell\ell$ transitions, allow for stringent tests of LFU, a fundamental symmetry of the Standard Model (SM) that assumes the universal coupling of electroweak gauge bosons to leptons. However, presence of New Physics beyond the SM can manifest in sizeable deviations from LFU.

These proceedings present a summary of recent measurements focused on branching ratios of $B\to K^{(\ast)}\ell^{(\prime)+}\ell^-$ and $B_s^0\to \phi \ell^{\prime +}\ell^-$ decays, with $\ell = e, \mu$, performed at LHCb.
The experimental techniques employed by LHCb are described, including the reconstruction and identification of the final-state particles, event selection criteria, and the estimation of systematic uncertainties. The discussed measurements constitute the most precise tests of LFU in $b \to s\ell\ell$ transitions to date and are compatible with the SM predictions.