Hi, I can’t wrap my head around it and kindly ask for your expert knowledge: what happened to the average foot soldier when his army lost (irl and westeros)? Get killed, just disarmed, forced to swear fealty, kind-of-enslaved for the winning army’s needs? What is his chance to see home again?

It’s a long quote, but it’s worth it:

“Broken men are more deserving of our pity, though they may be just as dangerous. Almost all are common-born, simple folk who had never been more than a mile from the house where they were born until the day some lord came round to take them off to war. Poorly shod and poorly clad, they march away beneath his banners, ofttimes with no better arms than a sickle or a sharpened hoe, or a maul they made themselves by lashing a stone to a stick with strips of hide. Brothers march with brothers, sons with fathers, friends with friends. They’ve heard the songs and stories, so they go off with eager hearts, dreaming of the wonders they will see, of the wealth and glory they will win. War seems a fine adventure, the greatest most of them will ever know.

“Then they get a taste of battle.

"For some, that one taste is enough to break them. Others go on for years, until they lose count of all the battles they have fought in, but even a man who has survived a hundred fights can break in his hundred-and-first. Brothers watch their brothers die, fathers lose their sons, friends see their friends trying to hold their entrails in after they’ve been gutted by an axe.

"They see the lord who led them there cut down, and some other lord shouts that they are his now. They take a wound, and when that’s still half-healed they take another. There is never enough to eat, their shoes fall to pieces from the marching, their clothes are torn and rotting, and half of them are shitting in their breeches from drinking bad water.

"If they want new boots or a warmer cloak or maybe a rusted iron halfhelm, they need to take them from a corpse, and before long they are stealing from the living too, from the smallfolk whose lands they’re fighting in, men very like the men they used to be. They slaughter their sheep and steal their chickens, and from there it’s just a short step to carrying off their daughters too. And one day they look around and realize all their friends and kin are gone, that they are fighting beside strangers beneath a banner that they hardly recognize. They don’t know where they are or how to get back home and the lord they’re fighting for does not know their names, yet here he comes, shouting for them to form up, to make a line with their spears and scythes and sharpened hoes, to stand their ground. And the knights come down on them, faceless men clad all in steel, and the iron thunder of their charge seems to fill the world …

"And the man breaks.

"He turns and runs, or crawls off afterward over the corpses of the slain, or steals away in the black of night, and he finds someplace to hide. All thought of home is gone by then, and kings and lords and gods mean less to him than a haunch of spoiled meat that will let him live another day, or a skin of bad wine that might drown his fear for a few hours. The broken man lives from day to day, from meal to meal, more beast than man. Lady Brienne is not wrong. In times like these, the traveler must beware of broken men, and fear them … but he should pity them as well.”

Odds are not good of getting home:

•  if you run, and you don’t get ridden down, maybe you make it home before you starve or die of infection or what have you, but you’re moving on foot and chances are you have no money for passage and you don’t own a map and you’re illiterate and you’ve probably never been more than ten miles from home your entire life so navigating by landmarks is going to be rough. 
• if you don’t, common soldiers don’t get ransoms (unless they worked out some bulk deal) and they’re not important enough to swear fealty, so odds still aren’t great. Medieval warfare had its share of massacres, especially of levies (for example during the Hundred Years War), with the idea being to reduce the enemy’s ability to fight back in the future. So you might well end up in a mass grave, like the ones from the Wars of the Roses that archaeologists keep digging up. You might get lucky and be thrown in prison, but your odds of coming up with enough money to get you out before the conditions in prison kill you are low.