There is a (perhaps) bewilderingly authoritative answer available! Oddly enough it is not very detailed in purely metallurgical terms, but it seems to offer all the general history and background anyone could want.
Haynes Publishing is a rather splendid British company that has specialised, since 1960, in detailed maintenance manuals for road vehicles. In recent years it has also extended to a few more or less whimsical publications, including the Millennium Falcon (Modified YT-1300 Corellian Freighter) Owner’s Workshop Manual (Haynes Publishing, Yeovil [Somerset, UK] and Newbury Park [CA, USA], 2011).
The book's first and most prominent publication announcement is ‘© 2011 LUCASFILM LTD. & TM. ALL RIGHTS RESERVED. USED UNDER AUTHORIZATION’. Lucasfilm’s specifically credited personnel are JW Rinzler (Executive Editor), Troy Alders (Art Director), Leland Chee (Keeper of the Holocron) and Carol Roeder (Director of Publishing). The (American) creators of the book itself are Ryder Wyndham (author) and Chris Reiff and Chris Trevas (illustrators), all alumni of many (at some stage) official SW texts across various media.
On the OP’s specific topic, we find that the YT-1300 is generally a very resilient and hard-wearing model, available from the Corellian Engineering Corporation (CEC) factory in innumerable custom-tweaked configurations, but all robustly armoured as standard (p16). The majority have cockpits with external line of sight, thanks to large transparisteel windows (pp6, 16). Some, however, are fully armoured: ‘CEC also offered options for a completely armoured ship with an “embedded” cockpit, located under a heavily shielded hull, without any windows or viewports.’ (p14)
With the general principle of durable armour thus established, the closest we come to specific metallurgical detail is a sectional diagram of the Millenium Falcon’s very own hull composition (p85). Here we find that the ship’s basic surface is defined by a grid-like reinforced frame (of unspecified composition). Upon that is mounted a layer of durasteel plate, in closely-fitting sections bonded by duralloy welds. A few mismatched plates seem to be fitted, perhaps as part of the proprietors’ active policy of keeping this particular ship looking as shabby as possible:
Han Solo does not bother to conceal the hammered-out dents,
durasteel-patched breaches, or epoxatal-filled cracks, and he leaves
most of the rust as is. [...] Han Solo and Chewbacca know that
pirates, thieves, and Imperial Customs agents are less attracted to a
ship that looks as if she’s ready for the scrapyard. (p84)
Another pertinent feature of this specific vessel (diagram again, p85) is an outer layer of ‘military-grade armour’, with distinctive interlocking edges:
Solo and Chewbacca have fused and welded sheets of scavenged duralloy
plating over most of the vital areas of the hull, providing
warship-grade protection for its engines and crew compartments. Much
of the plating came from the Imperial derelict Liquidator, a
Neutron Star-class bulk cruiser that was a casualty of the Battle of Nar Shaddaar.
With all that said, the manual (pp80–81) also makes quite a lot of the ship’s shielding capabilities:
Deflector shields are projected just a few molecules underneath hull
plating, but different power settings and configurations can extend a
shield farther away from the hull. [Ships of this size] have multiple
deflector shield generators dedicated to different areas.
[...]
Particle shielding repels solid objects such as space debris and
high-velocity projectiles. Because particle shielding completely
surrounds a ship, it must be temporarily turned off before a ship can
fire its own missile, launch an escape pod, or receive a shuttlecraft.
Ray shielding, also known as energy shielding, protects against
stellar and magnetic radiation, lasers, blasters, and other energy
beams. Ray shielding does not stop solid matter.
[...]
Han Solo refuses to discuss details, but rumour has it that he
‘acquired’ several military-grade deflector shield generators from the
Imperial maintenance facilities on Myomar.
Depending on what might be SW canon this week, then, one possible answer to this question is that the Falcon is made of durasteel and transparisteel, including military-grade armour, with duralloy welds and patches, oddments of epoxatal (overall with major or minor areas of something that routinely rusts), all built on a substructure that is in some sense reinforced.
Another might be that this physical structure is all well and good but would still have been instant toast in many of the Falcon’s specific experiences (including physical impact) were it not for the particle shielding, even if (as the manual states, p81), this (ahem) civilian ship ‘can endure such abuse for limited duration only, as the ship’s engines are not designed to provide the incredible power necessary to run military-grade shields continuously’.
A third could be that the Millennium Falcon’s... ummm... developed armour-and-deflectors capability means that for confrontational or simple impact purposes it is effectively composed of Imperial warship.
Maybe the Corellian shipyards have an advertising slogan along the lines of, ‘Even a Stormrooper couldn’t total one of ours.’
