How does one load it?

This may be up to change, but as of now, this is how it works : once all bullets are gone, the cylinder (which contains a barrel for each bullet) is ejected and another cylinder is put in its place.
That system is inspired by the HK P11 underwater pistol.

Depending on how fancy you want to get, this could also enable it to use a superposed load configuration (https://en.wikipedia.org/wiki/Superposed_load) to enable much higher magazine capacity. Admittedly, it would potentially involve deleting the hammer (although I could see it being retained for aesthetic + firing feedback reasons even if it doesn't actually do anything for the operation of the gun), but considering you could pretty plausibly (especially in the longer version) get capacities much closer to what you'd expect from a modern high-capacity handgun, and potentially eliminate even more moving parts that could get fouled by water ingestion, I can't see that being much of a downside.

Could be a mod with a mag built for that purpose yeah, since technically you could fit either of the mags drawn there into the gun. And the "hammer" on this design is actually the fire select button (up = safe, pulled down = fire) so that problem's already sorted out.

Perhaps that's even the purpose of the extra-long "magazine", to allow it to get both better muzzle velocity and higher capacity? At which point you could go for combining some real old-school firearms design with the revolver, by giving Lorenzo a holster stock (or, if you want a conventional holster for easier quick-draws, perhaps the detachable stock instead doubles as a holder for extra cylinders). That way, Lorenzo can combine sidearm and long arm into a single package - and, with the right cylinder, have a barrel long enough to justify the use of that bipod with it (especially if using superposed ammunition, as that also frees you from needing to use conventional cartridge designs and you can thus have near arbitrarily large powder charges behind a given projectile.

It's also worth noting that, assuming they all used the same firing hardware, you could potentially have "cylinders" for firing different types of ammunition - everything from 40mm grenade (potentially reloaded the way Russian 40mm underbarrel launchers are, ie., from the front with caseless rounds) to double-aught buckshot to more exotic stuff like beanbag rounds or the like. There's actually a nifty weapon concept I saw on Atomic Rockets (https://www.projectrho.com/public_html/rocket/sidearmslug.php#wiolant) that uses more or less the same concept for how its ammunition was stored, intended for use in space - and, as I suggest for Lorenzo's gun, it can change what type of ammunition it's firing just by swapping the magazine and barrels.

Definitely something I could delve into in the future. Dunno yet how far I'd go, but yeah I'm liking the idea.

As for the long barrel in the drawing here, this is a sniper variant based on the Manurhin MR73 Sniper, the french oddity I mentionned in the description. So, few ammo and long barrel.

Fair enough - I could also see a machine pistol/SMG/PCC cartridge: similar length to the sniper cartridge, but trades the high velocity of the sniper-cartridge's extended length rounds and longer barrel for far greater capacity. If the revolver itself can switch between a standard trigger for "semi-auto only" cartridges and a progressive trigger (think Steyr AUG) for full-auto capable ones, that would serve as the way Lorenzo chooses whether to fire single shots or full auto - otherwise, the cylinder would potentially just have a slow enough cyclic rate that Lorenzo could control the number of shots fired with single taps of the trigger.

EDIT: Apologies if I'm going entirely into the weeds, it's just that firearms design and technology is something I always tend to find fascinating, and it's honestly refreshing to see an artist taking design as seriously as you seem to. One minor suggestion for the design up top - possible integration of a laser sight alongside the underbarrel weapon light, ala the Laser Aiming Module off of the OTL MK 23 pistol. It probably wouldn't be too useful underwater, even with a blue laser, just due to diffraction and such, but above water it'd be potentially useful.

You're fine. I like to give attention to what I draw, including weapons, so it's nice to see someone being inspired by a design of mine and coming up with ways to expand on it.
I don't know if I'll keep all of your ideas, but they're worth exploring. Giving Lorenzo's handgun this level of modularity so it can become whatever weapon he needs to use appeals to me.

Rad. I like the multi-barrel design

Thx!

For a hot second I thought this was a before-and-after pic, like:

"Hah! Your little peashooter doesn't scare me, Lorenzo!"
Lorenzo's gun: *transforms*
"... okay now I'm scared WHAT THE SHI-"

Damn that scope is badass!