It's not uncommon for new writers, having written a book, to be terrified that someone is going to steal their ideas, write a different book based on them, and make a mint.
It may not be an uncommon fear, but it is an unreasonable one.
As I (and many other people) have said before: writing is the hard part, the ideas are easy.
I've spoken to writers who are too scared to send their manuscript to agents in case the ideas are stolen. While this is to some degree an understandable novice fear, it's also unfounded. 99% of manuscripts that go before an agent get such scant consideration that stealing of ideas - even if it were a thing - would not be possible. The great bulk of manuscripts are rejected after a page or two on the basis that the writing is not strong enough. Many that survive that first hurdle will fall fairly soon after on the basis that the pacing is off, or the voice weak, or the characters unengaging etc.
On the rare occasions that an agent gets to the end of a manuscript what they then do is consider where they might be able to sell it and if they believe it to be saleable they will offer to represent the author. What they won't do is try to get another author to stop what they're doing and write a new book using the ideas from the one they've just read.
(warning: magic system spoilers for two books follow)
The main (but still very small) danger is not that your ideas will be stolen but that if you delay sending out your manuscript because of this fear then eventually someone might just randomly come up with something similar.
Nearly twenty years ago I wrote an unpublished book, Blood of the Red (now available on Wattpad). The magic system is based on drinking the blood of dragons and thereby gaining powers that depend upon and are similar to the dragon's. So drinking the blood of a red dragon gives fire magics etc. Additionally, drinking the blood is fatal to almost everyone, but certain individuals can tolerate it.
In 2016 Anthony Ryan wrote the much better book The Waking Fire (first in a trilogy) in which the magic system shares exactly the same top-level description. Ryan didn't steal the idea from me - he had zero opportunity to do so, I didn't put the book on Wattpad until after his was published and I'd read it. It's just that if you wait long enough all possibilities will be explored.
But the bottom line is that if the most famous fantasy books in the world had not been published, and you came to me with the core ideas from The Lord of the Rings, A Game of Thrones, etc … neither I nor any other active fantasy writer would be snatching them out of your hands. We all have far too many ideas of our own that excite us. The difficult part of writing those books was not having the idea - it was putting words on pages in a manner that refuse to let readers go.
Once the book is published then of course the unfounded fear that other writers will steal the ideas can be replaced with the genuine knowledge that many readers will refuse to pay for your work and just take illegal copies off the internet.
So, bottom line, fearful writers: send your manuscript out to agents without fear. Nobody is going to steal your ideas, and they can't steal your skill.