Most CAMS and C3S data data is produced and archived not on a Cartesian grid (a two-dimensional rectangular grid), but on a reduced Gaussian grid - think of it as a globe with a series of evenly spaced data points along each parallel, and parallels spaced at regular intervals. So near the poles you have only few points along a parallel, but close to the equator you have about many data points along a parallel.
When you download CAMS data, C3S data and other data from ECMWF, you can specify to get the output data in a Cartesian lat/long grid and at a custom resolution.
In principle you can specify a higher output resolution than the default, but the system does not use more input points, it merely interpolates[1] the same data to a higher resolution. This makes the output look smoother, but does not increase the accuracy or the precision of the data.
For ERA-Interim the point interval on the native Gaussian grid is about 0.75 degrees (with the exception of Ocean-Wave data which are natively stored on the wave model’s reduced 1.0x1.0 degrees latitude/longitude grid). You can specify a custom grid on the data server web interface, or using the ECMWF WebAPI or using the MARS client (if you have access to it). On the web interface the default grid for ERA-Interim is lat/long, with a default resolution of 0.75x0.75 degrees (about 80km), approximating the irregular grid spacing on the native Gaussian grid.
For ERA5 test data the point interval on the native Gaussian grid is about 0.3 degrees. You can download ERA5 test data using Python and specify a custom grid and resolution in your script. You should set the horizontal resolution to no higher than 0.3x0.3 degrees (about 30km), approximating the irregular grid spacing on the native Gaussian grid.
[1] When data is interpolated, all continuous fields (e.g. precipitation, temperature) are interpolated by bilinear interpolation, and discrete fields (e.g. vegetation, precipitation type, soil type) and Wave 2D spectra are interpolated by nearest-neighbour. For more information about our grids and interpolations see in this presentation https://software.ecmwf.int/wiki/download/attachments/55122669/intro-interpolation-2016.pdf?api=v2
