Is there any difference between transferring DC over one 36 mm² wire or six 6 mm² wires?

It's not just losses. The wire is being heat up because of the I^2R, and the heat flows out through the surface. Six wires will have much bigger surface to cool down, hence you can have more current.

One thing to consider is that some electrical codes prescribe minimum diameters for certain wiring patterns - eg, a minimum diameter for the PEN conductor in TNC / TNC-S system - for safety against mechanical trouble (in the example of TNC, a wrecked PEN could have disastrous results if a heavy load with a "grounded" metal enclosure is connected). How this applies or does not apply to off-grid DC circuits is a matter of code, too.

One other (slightly off topic but important) thing to consider with heavy DC circuits: Anything that connects/disconnects (switches, fuses, automats, contactors, connectors) and is rated for xx amps AC at 250V is NOT automatically rated (or suitable or safe!) for xx amps DC at even lower voltages. The reason is AC-only-rated designs relying on the fact that eventual arcs will be interrupted quickly by AC zero crossing.

Several dozen meters @ 36mm2 is crazy, except for 10's kWp setup. I guess it is not the case. It is heavy, expensive and generally shows something is far from optimal. Except for marginal cases, you can both save a lot of money and get more power by keeping the wires between 2.5 and 6mm2, using a higher voltage solar panel stack and a corresponding inverter.