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If you use ERA5 data in a regular lat/lon grid, many software applications by default visualise the data as a continuous tiled surface, as in plot (a) on the right. In this case think of the data values as referring to the centres of the tiles.

However, you might prefer to think of the ERA5 data as point data the ECMWF interpolation software does not conserve area integrals, so interpolated values are best regarded as point values, with a regular spacing, as shown on the right in plot (b): here global ERA5 data was downloaded with a regular lat/lon grid and a resolution r of 0.3 deg, and plotted on top of a satellite image with 0.25 degree image resolution. The 'top left' ERA5 data point is always at Longitude=0 ; Latitude=90, with further grid points spaced by r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90.