basedir='C:\Users\Jacqueline\ERA5\Downloads\';
fn='ERA5_2010.nc';
N=2;
for i=1:N
fqn=fullfile(basedir,num2str(i,'%02d'),fn); % build fully-qualified filename
data=..... % read the file here
% do whatever with this dataset here before finishing loop to go to next
...
end
Above is simplest "dead-ahead" to read the fixed number of subdirectories and process each file in turn.
Enhancements begin with using dir to find out how many subdirectories there are so far and iterating over all of them or to build a list and let the user select desired ones.
To keep the data, use a cell array to hold each returned result so don't overwrite each time through the loop.
All sorts of other possibilities will come depending upon just what need to do...
