It varies greatly with location, era and crossbow strength.
A very light and simple crossbow might be hand spanned with your foot in the stirrup.
The strap and hook for slightly stronger for better leverage (and kinder on the hands).
Cocked with a goat's foot type separate leverage device for slightly stronger.
(Some crossbows build the reloading lever into the mechanism, a late Chinese crossbow built the lever and a magazine in.)
Heavier crossbows used a crank for mechanical advantage.
The heaviest and highest tech crossbows used a crank with a ratchet so you could pause without it back cranking on you.
I have seen other mechanisms used on light-medium style bows, both are modern inventions, but in the alternate history of a fantasy world, who knows?
A shotgun like mechanism built into the stock, where the firer shucks the stock like a pump shotgun to re-tension the string. (these bows tend to be on the light end of light)
A slight variant of that where the trigger catch mechanism is a little shuttle set in a rail system that slides forward with the string, with these, you pump back 18" or so, and via mechanical advantage gearing and a ratchet the shuttle moves back 3". . .it takes 5-10 pumps to get the string all the way back to firing position. (These tend to be on the strong end of light into middle weight bows)
A hinge and lock built into the stock, where you press a button and the long bar of the stock folds in half in a way that the string goes back to the trigger catch mechanism, then unfolding the crossbow back ads tension back to the string, using the shaft of the stock itself as the lever to add mechanical advantage. Once you get the stock back to straight it locks back into a single straight bar. (For a light bow this is surprisingly easy, for a heavier medium style bow it's hard work.)
I'll bet my list above is not exhaustive, especially with modern bows, people are amazingly inventive.
A lot of this is GM call, depending on the type of reloading mechanism and at what point the combatant pauses to do something else.
It's assumed that reloading would consist of:
1) moving the crossbow from fire position to cock position.
2) Cocking the crossbow by adding tension to the string and setting it on the trigger mechanism.
3) taking a bolt from it's container.
4) loading the bolt into the mechanism.
5) resuming firing position.
A pause on steps 1, 3, 4 and 5 would seem to be casual, and likely step 2 takes up the least of the time for a very light bow like a hand crossbow, but is the majority of the time for a heavy bow. . .and if a heavy bow uses a ratchet, it's intentionally designed to allow you to pause mid cranking. . .
That said, it also depends on the action, like if a caster wants to make a 1 hand cast mid reload is one thing, drawing your bastard sword to melee two handed then sheath and resume reloading is another. . .we book 20% to sheath or carefully put away a weapon, and 0% to drop it, and dropping your crossbow mid re-load may knock something loose, or in a bad situation get it wet/dirty or break it.
