Hi Jonathan Sun,
First, my pleasure to assist you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
Generally, when you assign folder permission to users to create notebook or other files into a folder in OneDrive for business, the users also have editing ability to edit/modify the content to created files and upload file into shared OneDrive folder as well.
Howevey, but it might be possible to disable the ability to user not to delete files in shared OneDrive folder. Via the below mentioned steps for reference:
OneDrive web page > Click on the Gear icon > OneDrive settings>Click on More Settings> Return to the old Site settings page >Click Site Permissions > Permission Levels > Edit> Scroll down to the bottom, click on the Copy Permission Level button> Give this new permission level a name, e.g. no delete, and uncheck the following 2 options. Then click on Create to save it.

As you mentioned, “where users can create OneNote Notebooks and add other file types into the folder, but they should not be able to edit or delete pre-existing files in the folder” as you know that if we assign permission to create a file into OneDrive shared folder, they can also edit ability. In this situation, I’m really afraid (first I humbly request please don’t mind and accept my apologies) your requirement is not meet in this specific scenario.
Meanwhile, regarding your concern, to improve the OneDrive permission view and this type of change has come up, you brought a brilliant point. As we listen to all feedback and consider them for additional feature/future versions of our products, we requested you send/provide feedback via our Microsoft OneDrive feedback portal. Your feedback helps us know which features are most important to you.
I appreciate your understanding and stay safe!!
Best regards
Waqas Muhammad