Alright there's a lot to unpack here. But the short answer is: armor is amazing, that's why the wealthiest people and elite fighters wore it. What sane person would choose to wear 30-60 lbs of steel on their body if it didn’t do anything but slow you down?

First things first, light, medium, and heavy armor isn't really how people thought of armor during the medieval periods. It's, I believe, in invention of D&D, though I could be wrong on that one. In any case, it's usually easier to conceptualize armor as being either soft armors or hard armors.

Soft armor were like gambesons or aketons, basically thick layers of cloth and/or leather on top of each other (interesting tidbit, among the historical recreators and scholars there’s a debate going on about how prevalent leather actually was in armor. While I’m personally on the rare side of the debate, depending on time period of course, it was used in armor. Didn’t look like biker jackets and fetish gear though, unlike what many games and shows would have you believe).

During the medieval period these gambesons were often the first real line of defense people had, it was cheaper to make, much easier to mass produce, so you’d see some of the common soldiers wearing it. And it could definitely save their life. Layers of padded cloth could turn a killing blow into a scratch, or even stop an arrow if you’re lucky. There’s an account of the 3rd Crusade that compared the infantry after being harassed by the Saladin’s forces as looking like pincushions as they moved with arrows sticking out of their armor. Though it’s ambiguous if they’re referring to the low infantry that could only afford the gambeson or the more wealthy who likely had cloth armor worn overtop their mail.

There have been a lot of tests in the last few decades to determine the effectiveness of various weaponry on armor. Unfortunately, a lot of these tests were done by people to “prove” how one weapon was better than another, or how one weapon could beat all armor without much in the way of unbiased scientific research. You’ll see someone use a longbow and punch through some thin piece of cooking metal and claim that therefore a longbow could punch just as easily through armor. That is not the case.

One has to remember that weapon technology and armor technology went hand in hand. As armor got better, weapons changed to match them, which then changed armor to deal with the weapons and so on. But as a general rule, the best armor of the day made you impervious from anything but the most direct blows toward the weak points in the armor.

While armor technology went through a lot of development over the period of time that we consider the medieval ages. I’m going to limit myself to full body mail that we see during the crusades, and then the full plate armor that isn’t really invented until the very end of the medieval period, but is what most people think of when talking about knights.

Mail, is largely immune to any weapon that cuts: swords, one-handed axes and the like. And with extra padding from the gambeson the mail has a pretty good cushion against a lot of bludgeoning weapons and piercing weapons like maces and daggers. Though these weapons could still wound or even kill. Despite the padding a solid hit with a mace or hammer on the head, could potentially bring the entire force of the blow down into the opponent’s neck. It would need to be a pretty direct hit, though. While a dagger or other such weapon could stick in between the rings of mail, or get pushed into a weak point where the mail isn’t as strong. Possibly going up under the neck armor to pierce into the throat. Or into the eye holes of the helmet to stab.
Arrows were a bit more of a problem than most melee weapons. One of the quick and dirty ways to see how effective armor was is to see what kind of weapons they had and when art depicts them as using some weapons. One of the easiest examples to illustrate this point is the huskarl of Norse and English fame. Now, huskarls were the wealthiest and theoretically the best warriors in their respective armies. In art they are depicted as being covered almost completely in mail armor and holding onto large shields when advancing toward the enemy, where they are in danger of being fired upon by their enemy. However, when actually engaged in melee combat, they would sling their shields on their back and fight with large two-handed axes as an anti-armor and anti-cavalry weapon. Now I want you to picture standing in the middle of a violent battle. Weapons are flying everywhere, swords, axes, spears, everything is whirling around much too quickly and chaotically for anyone to truly make out and keep their focus on. Someone attacking just out of eyesight could kill you before you could even know what’s going on. In such a situation any means of protection against this combat would be what you kept with you at all times. So to see them disregard their shield, one of their best forms of defense, and rely entirely on their armor to protect them. That speaks volumes about what they were worried about. Eventually armor, and weaponry got better, and we get to the High and Late Medieval period where plate harnesses were fully or at least almost fully developed.

You have heard of the battle of Agincourt. Where English and Welsh longbowmen beat back the advancing armored French knights. A lot of people have taken this to mean that the longbow could pierce through the armor like it was nothing. That isn’t really the case.

There are plenty of tests some of dubious quality to show that, some use the wrong arrow, some use the wrong bow, some the wrong armor. None are perfect, but this is the best one I’ve seen: https://www.youtube.com/watch?v=Ej3qjUzUzQg

So if longbows didn’t pierce through armor, how did the English win? Well, while one hitting a breastplate wouldn’t do much, an entire army of arrows can theoretically get into the weakpoints in the armor. There are accounts of the French knights lowering their heads as they charged because the volume of arrows shot at them would otherwise stick into their visors or breathing and eye holes. It’s also worth noting, that many of the French knights did make it to the English line, where they were stopped by the English knights fighting on foot, who hadn’t just charged over rough terrain into a constant rain of arrows.

In the terms of melee fighting, we see special anti-armor weapons being constructed like the pollaxe, one of my favorite weapons. It was designed specifically to be used while in full armor against opponents in full armor. And even then we have accounts of duels with the weapon continuing for long stretches of time where the armor of both contestants became battered beyond recognition but the men underneath were still fighting strong. The other powerful anti-armor weapon was the couched lance, being charged on horseback where the full weight of the horse and rider is placed on a single point to punch through the armor as best it can. This was so deadly that tournaments with these lances pioneered a thicker type of armor to protect the tournament knight. These armors could be twice as heavy as the usual field armor, and often didn’t allow much of the movement that would have been necessary to survive a battlefield, with pieces that restricted the arm and neck to move at all, all for the safety of the tournament knight.

But by this point the time of the near impenetrable armor was coming to an end. Now guns had lived concurrently with armor for hundreds of years, but as the power of guns grew, more and thicker armor needed to be developed to stop it. This was when the concept of bullet-proof came about. In order to prove an armor was immune to these weapons, the armor-smith would show off the armor take a pistol and shoot it point blank in the chest. The small dent the bullets made was the proof that the bullet couldn’t penetrate it. Here’s an example: https://i.pinimg.com/originals/d1/ce/c4/d1cec4d92b9afc9a92494fac016226e9.jpg

But we start to see armor getting thicker and heavier during this period to continue its dominance to the point that only the wealthy could afford it at all. And medieval style armor slowly disappeared, though armor itself isn’t gone. Even our current soldiers and police forces wear armor to defend against shrapnel and small arms fire.

But in brief. No. A movie or tv hero placed into an actual medieval battlefield would have their sword bounce off their opponent’s armor, and then they’d die. Likely from an attack they never even saw coming. Because they didn’t wear any real protection, and if they couldn’t afford it, weren’t standing in a protective line with their friends holding up their shield to protect them.