They are usually made out of iron and you stuff the small branches into bottles. You can try using an actual large branch screwed to a wooden base. Not sure how long it would last outdoors though.
Here's a good tutorial: https://dengarden.com/gardening/Bottle-Tree-For-My-Garden-Reuse-of-Junk
Like this? https://www.hometalk.com/11701029/coca-cola-glass-bottle-christmas-tree-craft
I just saw some great ideas on Hometalk -- did you check around using the search bar at the top left?