I've made this mini crossbow for a challenge at creality cloud but didn't uploaded it, but I wanted to post it somewhere anyway so here is it
It can be printed in PLA ABS or PETG, and I use spare filament to fit the parts together, to do it just assemble the pieces and put a piece of filament on the holes, leave about 5mm on each side and heat the end, and press it against the parts, to form almost like a nail head
for the bow string just get some string that have enough clearance to go through the trigger/body parts
the projectile can be made with a toothpick flatted in one of the sides or even with some raw filament
to operate force the trigger back, and pull the string till the trigger catch it, the slide the toothpick on the top until it fit on the stopper near the trigger aim and fire!
be careful with rebounds, the toothpick is not stabilized and can be deflected and hit eyes other sensible stuff