How to combine files in subdirectories in macOS
An easy way to do it would be use the Terminal, and write command as follows:
mkdir 2013

This create a folder named 2013 after the year.
mv *-2013/* 2013

This moves all the files from the folders those names end with &quot;-2013&quot; into the folder named 2013.
If you have a large number of years to do this for, consider using a simple bash script with a for loop - something like this:
for year in {2000..2020} 
do
  mkdir $year
  mv *-$year/* $year 
done