Recently, I used the transpile() function with the level 3 preset pass manager to compile a very long and complex circuit on a real device coupling map. I was curious to see how much the depth of the circuit would increase due to the inevitable swaps. The result was quite strange, the depth of the transpiled circuit was an order of magnitude lower than the original circuit.
When I drew the transpiled circuit I found that it was composed of some strange multi-qubit gates labeled as 'unitary' (see picture below), while most of the two-qubits gates from the original circuit were gone.
I was wondering what these unitaries could mean.