ERA5 was produced using 4D-Var data assimilation in CY41R2 of ECMWF’s Integrated Forecast System (IFS), with 137 hybrid sigma/pressure (model) levels in the vertical, with the top level at 0.01 hPa. These are hybrid pressure-sigma levels, which are pure sigma levels near the surface, pure pressure levels near the top of the model and a mixture in between.
Upper air data for ERA5 is provided on various level types, including pressure levels, where the data has been interpolated from the model levels. The pressure level data goes from 1000 to 1 hPa. If you want to look at pressure levels above 1 hPa you can use the model levels, which are pure pressure levels there, see the model level definitions at https://www.ecmwf.int/en/forecasts/documentation-and-support/137-model-levels. However, beware that not all the parameters provided on pressure levels are available on model levels, see the ERA5 Data Documentation (Parameter listings) for more details.
Note, the model level data is not listed in the Climate Data Store (CDS), but you can access it from our archive (MARS) using the CDS API - see How to download ERA5 (Step C in section 3).