Q.elderberry plants
The fruit of elderberry plants sit on the ends of very fine short stems which are attached to much larger long stems. I have heard that elderberry stems are poisonous. Are both the very thin and the thick stems harmful when making jelly?

Only the berries are used for cooking, like making jelly. They lose their toxicity when cooked properly.
The stems should not be used or consumed.
Here is a link with more information.
https://www.gardeningknowhow.com/edible/fruits/elderberry/planting-elderberry.htm