The crossbow is a strong bow, which is mounted transverse on the cob with arrow slot. This allows to use two hands and two feet to strain (sometimes with levers and pulleys) to tighten and secure the string on the cob. A (rotating) trigger mechanism holds the string. A trigger keeps the pawl locked. Because you do not have to hold the tension and stress yourself constantly you can target calmer and better, while shooting with a lot more power.
The arrows shall be heavy enough to absorb the energy sufficiently.
A crossbow you can, once stretched, keep stretched for long without effort. And by cocking devices and pulling techniques (using foot or mechanisms) arrows were fired with much more power.
For a crossbow you use a strong tendon. I have one that consists of 25 wires. Wear-prone spots like ends and center are wrapped (a ‘whipping'). You have to rub frequently with wax.

The idea of ​​a crossbow probably came from the bow trap, on which a mounted, bent bow went off as prey eat bait and thereby took away a trigger or did slip a string holder.

Or I find a way or I will make a way.