There is always more to say, but this question has come up a few times before and you might like to check out some earlier responses while you wait for fresh answers to your question. The long and short of it is that there is little to no reliable evidence for the existence of "ninjas" in the modern sense of what the term typically means; the concept of ninjas as uncatchable, super-skilled master assassins is a modern invention.

There's a lot of pop culture nonsense about ninjas. What books and resources would you recommend that show what real, historical ninja/shinobi were actually like.

Best nonfictional sources/books on accurate historical info about ninjas and their function/influence on Japanese society?, both with u/ParallelPain