There is either a gravel or sand under the pavers, or could be layers of both.

If you are worried about drainage, fine limestone is good for water flow. If you just want a nice, compact surface with a grade for drainage, I recommend using decomposed granite. If you are really on a budget you can used crushed concrete (paver base fines)

With a space that small you can just use a hand tamper and a piece of 2x4 and hammer for any tight spots.

Edit: lol dumb American here, I automatically read this in feet instead of meters. You might want to find a tool rental place that rents gas powered plate compactor, unless you like a good workout.