SharePoint is a enterprise document management system, that got robust features and permissions granularity. The files in SharePoint are stored in an application that is called Document Library.
The common element among SharePoint Document Library(DL) and other Online File Storage(OFS) is that they store files and that’s where the similarities end.
The answers to the questions may help to understand the difference between DL and OFS.
How do I share the files with a group of users ?
Document Library
Create a SharePoint Group, Add all users into it and Grant Permissions
Create a share link with expiry date and password enabled
Separate links for people to view and for another set of users to edit the files within the folder that is shared or a single file can be created
Online File Storage
Enter email of the users to which you want to Share the document with.
How do I keep the files secure so that they aren’t shared accidentally ?
Document Library
Adding sensitivity labels to the documents allows to restrict documents from being shared and block access even if the documents are copied.
As an example, A document requiring that an internal user needs to sign-in with an organizational account to view the document.
Online File Storage
Review the permissions, but once the file is shared the users can copy and share the file.
The only way would be to avoid sharing files altogether