  Occultations are windows of opportunity to indirectly peek into the dayside
atmosphere of exoplanets. High-precision transit events provide information on
the spin-orbit alignment of exoplanets around fast-rotating hosts. We aim to
precisely measure the planetary radius and geometric albedo of the ultra-hot
Jupiter (UHJ) KELT-20 b as well as the system's spin-orbit alignment. We
obtained optical high-precision transits and occultations of KELT-20 b using
CHEOPS observations in conjunction with the simultaneous TESS observations. We
interpreted the occultation measurements together with archival infrared
observations to measure the planetary geometric albedo and dayside
temperatures. We further used the host star's gravity-darkened nature to
measure the system's obliquity. We present a time-averaged precise occultation
depth of 82(6) ppm measured with seven CHEOPS visits and 131(+8/-7) ppm from
the analysis of all available TESS photometry. Using these measurements, we
precisely constrain the geometric albedo of KELT-20 b to 0.26(0.04) and the
brightness temperature of the dayside hemisphere to 2566(+77/-80) K. Assuming
Lambertian scattering law, we constrain the Bond albedo to 0.36(+0.04/-0.05)
along with a minimal heat transfer to the night side. Furthermore, using five
transit observations we provide stricter constraints of 3.9(1.1) degrees on the
sky-projected obliquity of the system. The aligned orbit of KELT-20 b is in
contrast to previous CHEOPS studies that have found strongly inclined orbits
for planets orbiting other A-type stars. The comparably high planetary
geometric albedo of KELT-20 b corroborates a known trend of strongly irradiated
planets being more reflective. Finally, we tentatively detect signs of temporal
variability in the occultation depths, which might indicate variable cloud
cover advecting onto the planetary day side.
