Assume we have a combat taking place in a big, open, flat field with no furnishings, so we're just dealing with the characters themselves. In this case, how does cover work?
Do enemies provide cover from the heroes for other enemies? Do friendly allies provide cover from enemies for each other? Do size or status effects affect anything? How much cover is granted (½, ¾, full) and for what things? Can anyone Hide (halflings?), and what does that help with?
Creatures technically always provide at least half cover:
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend (BD&D p74).
So basically, if you're shooting past someone (or even in melee on the other side of a creature), the attack is made against half cover.
I say technically, because my guess is that most DMs don't remember this and so it's left largely unenforced, largely on the ally side. I personally rationalize this by figuring that characters who are used to fighting together are going to stay out of each other's way. However, if you're looking for the by the book answer, all creatures provide half cover.
If you are using a grid and miniatures it is pretty easy, if another character is between an attacker and the target then the target gets cover, how much is a DM evaluation of the range, sizes of the creatures, etc.
If using theater of the mind, then the DM needs to be very descriptive about who is blocking/covering who. Given how the movement rules work in 5e it is pretty easy for a character to shift to the side, take a shot, then shift back in order to get around something in their line of sight so I generally only consider things near the target that might block line of sight, not things (other than a hostile enemy, perhaps) near the attacker.
A good rule of thumb is that if a creature is "in the way" somehow, grant the target a +2 to AC for half cover. I've been using this for ranged PCs shooting at a target in melee with another PC (and the PC is between the attacker and the target). I've been reserving 3/4 cover (+5 AC) for targets within structures, though I suppose a large animal (like a horse) in the line of sight of a medium target might also qualify.
