Explain grafted trees to me like I’m 5

I only learned fairly recently that ‘grafted’ didn’t mean pruned 🙂

I get the very basic idea that the top of one type of plant is somehow attached to the roots of another.

What are the implications of that? A shrub looks like a tree instead?

How are they attached? Manually, after germination? Frankenstein style?

Should the tag on the plant for sale specify what type of roots are attached or does only the top matter?

If I just let the rootstock send up shoots will it actually grow the type of tree that is the root stock or will it just be long shoots of leaves with nothing distinguishable?

Teach me please!