Hi. How do you cope with piecemeal armor in the rule system? I mean, a PC wants to buy only  armor fot the legs. How does it cost? Does it give AC bonus? And what about called shots? I know that D&D is not a game that deals with so much detail, but I do not what to do if a player wants to buy piecemeal armor. Any suggestions?

Is the player looking to create a specific aesthetic for their character?  Or are they looking to get higher AC for a lower cost?

If the former?  Let them buy whatever they'd like, and tell them they get the AC of a specific suit of armor that feels roughly analogous to what they want, or the AC of whatever they're wearing on their chest.  A PC that wants to buy metal armor for their feeties but wear only leather on their chest (odd choice, but sure) gets the AC of wearing Leather but the aesthetic of wearing whatever they like elsewhere.  Perhaps they get circumstantial bonuses as appropriate - a PC who spends gold on metal sabatons despite being a leather guy might get advantage on saves against floor spike traps, or gets to use a higher* AC against effects that specifically target the legs, but I'd make it plain from the start that these bonuses will be circumstantial, and the circumstances will be rare.  It's more of a "nice bonus" thing than a reason to do it.

If the latter?  Nah.  Piecemeal armor, by its nature, is less protective than a full set of fitted armor designed to be worn together by a specific person.  If they really want to push it, then all piecemeal armor qualifies as Hide armor, no matter what it's actually made of.  Or Leather, if you're feeling generous and want to let someone retain light armor Dex privileges whilst being a butt with piecemeal kits.

They don’t do piecemeal armor in 5e. That was older editions where one could get greaves etc. piecemeal, not this edition.

Considering that an entire gambeson (aka Padded) is only +1 AC over naked, if they added a little to it like greaves and bracers, then yeah, I would consider Yurei’s suggestion and maybe treat it as Hide, but add stealth disadvantage. (Although, it makes absolutely no sense whatsoever that a gambeson would be un-stealthy. 🙄)

Yeah, Padded armor has never made any damn sense to me.  But then again, I actually have some basic knowledge of medieval armor and weaponry from sources other than fantasy books, so someone saying "your gambeson makes a ton of noise/makes it hard for you to move stealthily!" in a game where actual metal armor doesn't do the same is just...what?  What?

But yeah.  Frankly, after thinking about it, I'm inclined to just say "you get the AC of whatever you're wearing on your chest; the rest you can season to taste" and let that be the end of it.  if someone tries to piecemeal 'chest', then they get the lowest AC of whatever they're piecing in their meal.  If they complain?  They get the whackin' stick and a lesson on medieval armor.

And considering that even leather armor went out in the Bronze Age....  While the Gambeson stayed technically relevant from before the Dark Ages through the Renaissance period, so, for like 1,000 years.