chainmal mostly hangs from the shoulders and the belt. The thing is, Maille is flexible, but not unlimited flexibility. good Maille is tailored to the wearer. If the clothing underneath, i.e. padded stuff, is needlessly thick, you’d end up losing mobility.
And as per full palte, sure, it doesnt cover 100%, that’s where the PARTIAL Maille go into play. I.E. Maille Sleeves, or ‘Voiders’ sewn to the arming jacket under the arms etc.
as per shirt or skin. Only an idiot would were any armour over bare skin. That being said, the normal everyday clothing of times past usually was far more substantial than our modern thin T-Shirt. A full sleeved linnen shirt or under tunic, and over that usually a sturdy wool tunic, or in later times doublets etc. Most of these outer garments were made of strong cloth and lined. partially padded and doubled up for fashion etc.
By the 17th century the Armour for massed troops had evolves such, that the pieces of srmour did not need to be tied to a specially deviced under jacket, but rather were attached to each other…and commonly worn over the soldiers normal clothing.
The point being, a full suit of fully padded textile armour underneath a full suit of Maille, underneath a full suit of armour insn’t quite the way it seemingly was done ‘back then’
If you had only extile armour, it’d be purpose made and thick. If you wore Maille, the padded clothing underneath would’ve been purpose made and not as thickly padded as the former standalone stuff. And if you wore full plate, well you wouldn’t want to waste your strength carrying around superflous MAille, so you’d wear partial Maille Bits, over or attached to light padded clothing.