I use oak leaves that have dried on the tree, I pick them in the fall, and they add tannins and acidify water, as well as providing cover for fish that use leaf litter in the wild. In a closed system, if you leave them to decompose, you have problems. The same would be true with taking leaves from a stream, because different tree species have different chemical properties, and you want to control what happens with your water. Leaves are also used by other creatures you would not want to have in your tank, so dry leaves are best.
As an example, maples also add tannins, but they rot so fast they are a negative. You have to learn how to do this.
I use alder cones, though I am less impressed by them, to make a tea to add tannins.
You have to want tannins, which stain the water brown.
For hardwater, tannin hating fish I have added pieces of limestone to tanks. It releases minerals into the water. That would be awful for a cardinal tetra, but good for a platy.
I also collect all my own rocks, and soak pieces of hardwood to have wood decor. Softwood would be a disaster, and rocks that release minerals would mess up my rainforest tanks.
Most short videos are untrustworthy because they don't give background. I use field guides for rocks, and only put natural or unnatural things into a tank if I have a reason to. Nature is very diverse and complex and you have to respect that.